soc: arm: stm32wl: wakeup from stop clock selection based on sysclk

When exiting Stop mode, if system clock is MSI, MSI oscillator is
selected as wakeup from stop clock; otherwise HSI16 oscillator is
selected.

It is otherwise reconfigured as MSI just after, but it slightly
increases the wake-up time and power consumption.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>