dts: stm32l4: Use HSI48 as 48MHz clck source when available

On STM32L4 that probvide HSI48 clock, use it as 48MHz domain clock.
This impacts following devices:
-SDMMC
-RNG
-USB
Otherwise, when HSI48 is not available MSI is used.

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