refactor(st-clock): update STGEN management

Rework STGEN config function, and move it to stm32mp_clkfunc.c file.

Change-Id: I7784a79c486d1b8811f6f8d123e49ea34899e9b6
Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>
4 files changed