drivers: serial: stm32: Fix compile issues
drivers: serial: stm32: Fix compile issues

The stm32 uart driver fails to build on certian platforms due to
changes introduced by:

commit 3c18bcbf77da62f9f8fef0352520d7f1aa268dc1
Author: Francois Ramu <francois.ramu@st.com>
Date:   Wed Jan 27 10:27:33 2021 +0100

    drivers: serial: stm32 restore uart after lowpower

Fix this by adding some ifdef's around the code that is specific to the
given platforms that the code works on.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr