gpio: Support drive strength configuration.
gpio: Support drive strength configuration.

Provide a mechanism to configure different pin drive strengths on
hardware with that capability.

Configuration flags are provided to select each of DFLT, ALT and
DISCONNECT drive strengths independently for low and high outputs.

This provides sufficient flexbility to configure all of the drive
strength capability in at least nRF5 hardware.

The flags are chosen such that in the absence of a drive strength
flag a driver selects DFLT drive strength.  This ensures that an
existing application that omits a flag continues to observe the
preexisting behaviour.

The behaviour of the drive strength flags is documented such that a
driver for hardware that does not support a particular drive strength
will simply defalt to the standard drive strength for that hardware.

Change-Id: I9894cc5e739a1899a4ecf795f2a5980b95b0c7a0
Signed-off-by: Marcus Shawcroft <cf6354583ee83038f2010cbcbe1b1f2adb85107d@arm.com>

ldts/zephyr,holtmann/zephyr,GiulianoFranchetto/zephyr,bigdinotech/zephyr,bboozzoo/zephyr,sharronliu/zephyr,pklazy/zephyr,Vudentz/zephyr,fbsder/zephyr,pklazy/zephyr,aceofall/zephyr-iotos,bigdinotech/zephyr,runchip/zephyr-cc3200,rsalveti/zephyr,mbolivar/zephyr,mbolivar/zephyr,holtmann/zephyr,galak/zephyr,bboozzoo/zephyr,fractalclone/zephyr-riscv,bigdinotech/zephyr,runchip/zephyr-cc3220,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,bboozzoo/zephyr,kraj/zephyr,explora26/zephyr,aceofall/zephyr-iotos,runchip/zephyr-cc3220,galak/zephyr,holtmann/zephyr,finikorg/zephyr,pklazy/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,runchip/zephyr-cc3200,bigdinotech/zephyr,tidyjiang8/zephyr-doc,fbsder/zephyr,punitvara/zephyr,tidyjiang8/zephyr-doc,punitvara/zephyr,sharronliu/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,tidyjiang8/zephyr-doc,explora26/zephyr,runchip/zephyr-cc3220,explora26/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,Vudentz/zephyr,kraj/zephyr,galak/zephyr,runchip/zephyr-cc3220,holtmann/zephyr,nashif/zephyr,fbsder/zephyr,Vudentz/zephyr,zephyriot/zephyr,zephyriot/zephyr,ldts/zephyr,nashif/zephyr,zephyriot/zephyr,runchip/zephyr-cc3200,kraj/zephyr,sharronliu/zephyr,pklazy/zephyr,punitvara/zephyr,fbsder/zephyr,aceofall/zephyr-iotos,zephyrproject-rtos/zephyr,holtmann/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,bboozzoo/zephyr,erwango/zephyr,fbsder/zephyr,erwango/zephyr,aceofall/zephyr-iotos,zephyrproject-rtos/zephyr,bboozzoo/zephyr,tidyjiang8/zephyr-doc,aceofall/zephyr-iotos,Vudentz/zephyr,zephyriot/zephyr,galak/zephyr,ldts/zephyr,rsalveti/zephyr,kraj/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,zephyriot/zephyr,rsalveti/zephyr,explora26/zephyr,rsalveti/zephyr,sharronliu/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,fractalclone/zephyr-riscv,runchip/zephyr-cc3220,sharronliu/zephyr,erwango/zephyr,fractalclone/zephyr-riscv,explora26/zephyr,fractalclone/zephyr-riscv,finikorg/zephyr,pklazy/zephyr,runchip/zephyr-cc3200,GiulianoFranchetto/zephyr,bigdinotech/zephyr,fractalclone/zephyr-riscv,ldts/zephyr,kraj/zephyr,nashif/zephyr,Vudentz/zephyr,mbolivar/zephyr,galak/zephyr,rsalveti/zephyr,erwango/zephyr,tidyjiang8/zephyr-doc,runchip/zephyr-cc3200,erwango/zephyr