ARM: at91/dt: sama5d3 xplained: add mmc0 vqmmc entry

The SAM5D3 Xplained device tree is missing the vqmmc node which is
tied to 3.3V on the board. Add this to avoid the kernel warning that
there is no vqmmc node.

atmel_mci f0000000.mmc: No vqmmc regulator found

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>