Rework the way SDMMC clock get rate is done in a more
generic way :
_ Add stm32_clk_get_pllsai_rate() which give the PLLSAI
indicated output rate.
_ Add stm32_clk_get_pllsai_vco_rate() which give the VCO
internal rate.
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>