boards: stm32f746g_disco: Update board doc for sdmmc
boards: stm32f746g_disco: Update board doc for sdmmc

Add entries to features and pinmux tables to reflect support for sdmmc.

Signed-off-by: Anthony Brandon <6e1a438cfe5a6c9e2165665f8c2258849ccc43f0@amarulasolutions.com>

galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr