soc: stm32l0: Clear LPSDSR when exiting STOP mode

Clear the LPSDSR bit of PWR_CR to restore the voltage regulator
mode when exiting from STOP mode. Leaving LPSDSR set can cause
cause stability issues because the MCU will enter low-power sleep mode
instead of normal sleep mode when the core idles and this is not what
the rest of the implementation expects.

Signed-off-by: Alexander Mihajlovic <a@abxy.se>