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>