clk: clk_stm32fx: add clock configuration for mmc usage

MMC block needs 48Mhz source clock, for that we choose
to select the SAI PLL.
Update also stm32_clock_get_rate() to retrieve the MMC
clock source needed in MMC driver.

STM32F4 uses a different RCC variant than STM32F7. For STM32F4
sdmmc clocks bit are located into dckcfgr register whereas there
are located into dckcfgr2 registers on STM32F7.
In both registers, bits CK48MSEL and SDMMC1SEL are located at
the same position.

Signed-off-by: Christophe Priouzeau <christophe.priouzeau@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Vikas Manocha <vikas.manocha@st.com>