The clock source selection of LPUART depends on System clocks but also on
the serial baudrate. There is a specific computation done in serial driver
targets/target_STM/serial_api.c
At first start-up the LPUART1 clock selected in SetSysClock was anyway
overridden by the serial driver, so this was of no effect. But in case
of deep sleep SetSysClock is called again, while the driver isn't, so
SetSyClock was corrupting the serial clock configuration.
So let's remove these few lines of code which are causing trouble.
For targets L496 and L5.
Taking into account device TRNG in L5 configuration