ARM: dts: at91: sama5d2 Xplained: Describe the SD card power supply

As the SAMA5D2 SDHCI controller works with an external power supply,
describe the power supply for the SD card slot. This makes it possible
to use mmc power sequences, in the case of external SDIO modules.

Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>