uart: make poll_in and poll_out mandatory
uart: make poll_in and poll_out mandatory

Polling in/out are basic UART functions that are supposed to
work on every UART controllers. Therefore, both poll_in and
poll_out are now mandatory. The API no longer checks if
they are defined before calling. This is to avoid the if
statements so to speed up things a bit.

Change-Id: I3bd1e7a2dfa5405699ebaec3eb34cfc0434b0073
Signed-off-by: Daniel Leung <d94e04d205b5962d7873a139ef298b03c1717f72@intel.com>

32bitmicro/zephyr,mirzak/zephyr-os,mirzak/zephyr-os,ldts/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,runchip/zephyr-cc3220,zephyriot/zephyr,erwango/zephyr,GiulianoFranchetto/zephyr,pklazy/zephyr,fractalclone/zephyr-riscv,fbsder/zephyr,runchip/zephyr-cc3220,holtmann/zephyr,finikorg/zephyr,nashif/zephyr,rsalveti/zephyr,explora26/zephyr,nashif/zephyr,aceofall/zephyr-iotos,fractalclone/zephyr-riscv,coldnew/zephyr-project-fork,punitvara/zephyr,32bitmicro/zephyr,tidyjiang8/zephyr-doc,pklazy/zephyr,Vudentz/zephyr,zephyriot/zephyr,rsalveti/zephyr,bboozzoo/zephyr,zephyriot/zephyr,tidyjiang8/zephyr-doc,runchip/zephyr-cc3200,punitvara/zephyr,jamesonwilliams/zephyr-kernel,kraj/zephyr,Vudentz/zephyr,mirzak/zephyr-os,nashif/zephyr,sharronliu/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,jamesonwilliams/zephyr-kernel,punitvara/zephyr,ldts/zephyr,Vudentz/zephyr,erwango/zephyr,explora26/zephyr,sharronliu/zephyr,rsalveti/zephyr,32bitmicro/zephyr,bigdinotech/zephyr,coldnew/zephyr-project-fork,bigdinotech/zephyr,runchip/zephyr-cc3200,finikorg/zephyr,fractalclone/zephyr-riscv,nashif/zephyr,fractalclone/zephyr-riscv,runchip/zephyr-cc3200,explora26/zephyr,finikorg/zephyr,32bitmicro/zephyr,aceofall/zephyr-iotos,mbolivar/zephyr,kraj/zephyr,coldnew/zephyr-project-fork,Vudentz/zephyr,rsalveti/zephyr,fractalclone/zephyr-riscv,erwango/zephyr,bigdinotech/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,runchip/zephyr-cc3220,fbsder/zephyr,bboozzoo/zephyr,pklazy/zephyr,galak/zephyr,bboozzoo/zephyr,aceofall/zephyr-iotos,erwango/zephyr,bboozzoo/zephyr,erwango/zephyr,galak/zephyr,punitvara/zephyr,aceofall/zephyr-iotos,galak/zephyr,aceofall/zephyr-iotos,mirzak/zephyr-os,Vudentz/zephyr,pklazy/zephyr,sharronliu/zephyr,tidyjiang8/zephyr-doc,ldts/zephyr,holtmann/zephyr,galak/zephyr,fbsder/zephyr,holtmann/zephyr,runchip/zephyr-cc3200,GiulianoFranchetto/zephyr,jamesonwilliams/zephyr-kernel,jamesonwilliams/zephyr-kernel,zephyriot/zephyr,GiulianoFranchetto/zephyr,holtmann/zephyr,sharronliu/zephyr,bigdinotech/zephyr,pklazy/zephyr,bboozzoo/zephyr,fbsder/zephyr,mbolivar/zephyr,mbolivar/zephyr,coldnew/zephyr-project-fork,zephyriot/zephyr,mbolivar/zephyr,kraj/zephyr,mirzak/zephyr-os,sharronliu/zephyr,holtmann/zephyr,coldnew/zephyr-project-fork,tidyjiang8/zephyr-doc,galak/zephyr,runchip/zephyr-cc3220,zephyrproject-rtos/zephyr,explora26/zephyr,runchip/zephyr-cc3200,explora26/zephyr,Vudentz/zephyr,ldts/zephyr,runchip/zephyr-cc3220,bigdinotech/zephyr,punitvara/zephyr,fbsder/zephyr,tidyjiang8/zephyr-doc,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,jamesonwilliams/zephyr-kernel,rsalveti/zephyr,kraj/zephyr,finikorg/zephyr,32bitmicro/zephyr