ARM: at91/dt: sama5d3 xplained: add fixed regulator for vmmc0

Add fixed regulator for vmmc0 and attach the vmmc for it to the mmc0
node on the SAM5D3 Xplained board. This will remove the following
warning from the kernel:

atmel_mci f0000000.mmc: No vmmc regulator found

Note, atmel_defconfig will need fixed regulator support enabled if this
is to be used properly.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
[use a fixed regulator instead of gpio one]
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>