soc: arm: infineon: 4xxx: Fix UART comms during sleep
soc: arm: infineon: 4xxx: Fix UART comms during sleep

In normal operation the clock for peripheral devices is derived from
the PLL clock.
During sleep, the xmc4xxx can switch its system clock source (f_sys)
from the PLL clock to an f_ofi clock. Switching to the f_ofi clock
is the default behaviour which breaks UART comms.

Use the PLL clock source during sleep instead to fix the issue.

Signed-off-by: Andriy Gelman <9b800b50c068448b0612a5bdc33ac199b44926a7@gmail.com>

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