dts: stm32: Configure SDMMC 48MHz domain clock

Provide SDMMC domain clock, when required (because it is common)use
the same as the one selected by USB and RNG.
Otherwise, when available use HSI48, otherwise use the most handy (MSI,
sysclk ...).
PLLSAI is not used as not implemented for now.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>