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 <andriy.gelman@gmail.com>