drivers: serial: Add power management to nRF UART driver
drivers: serial: Add power management to nRF UART driver

This patch changes the way the transmitter is handled in the UART
driver, so that it is activated only when there is something to send.
The current UART API does not allow to disable RX completely, since
the poll_in function description implies that UART must continuously
listen. To provide a way of disabling the entire UART and lowering the
current consumption, this patch adds the power management to the UART
driver. When instructed to enter any of the power saving states,
the driver will disable the UART, and it will enable the UART back
when it is switched again to the active state.

Signed-off-by: Andrzej Głąbek <6788b8d556a8a447102338fc996c42180ecff44e@nordicsemi.no>

kraj/zephyr,ldts/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,explora26/zephyr,Vudentz/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,Vudentz/zephyr,nashif/zephyr,explora26/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,Vudentz/zephyr,explora26/zephyr,galak/zephyr,nashif/zephyr,kraj/zephyr,galak/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,explora26/zephyr,ldts/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,punitvara/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,nashif/zephyr,galak/zephyr,explora26/zephyr,Vudentz/zephyr,galak/zephyr,punitvara/zephyr,finikorg/zephyr,finikorg/zephyr,kraj/zephyr