stm32mp1: clk: add common function pll_get_fvco

the function compute the VCO PLL freq, used in
- stm32mp1_read_pll_freq()
- pll_set_rate()

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
# Conflicts:
#	drivers/clk/clk_stm32mp1.c
1 file changed