refactor(st-clock): support deactivated STGEN in stm32mp_stgen_config

Update function stm32mp_stgen_config() to support deactivated STGEN
when frequency is 0, for example on STOP2 exit for STM32MP25.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Change-Id: Id371c4602a614bbfa0ecc7ce2d2e0ac5261e1d52
1 file changed