boards: arm: stm32l496g_disco: enable sdmmc support

Update the dtsi for stm32l471 (which the higher SoCs are based on) to
support the stm32-sdmmc disk access device. Enable the device for the
stm32l496g_disco board, and update the pinmuxing.
Note that the stm32l496g_disco board also has a card detect gpio
(MFX_GPIO8), but this is not supported yet. When not specified the
driver will assume a card is present.

Signed-off-by: Anthony Brandon <anthony@amarulasolutions.com>