drivers: clock_control: stm32u5: Get MSIS freq from SoC registers

MSIS frequency at boot time can be different from the one we intent to
set from device tree configuration.
In order to avoid issues, read MSIS configuration from registers to get
the actual freq rather than the devicetree one which may be not yet
configured (which is the case at startup).

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>