Move the MMC configuration from the board C file to devicetree.

The 'power' GPIO was removed and instead the vmmc regulator is used,
to follow the changes introduced in the jz4740-mmc driver.
Signed-off-by: NPaul Cercueil <paul@crapouillou.net>
Signed-off-by: NPaul Burton <paul.burton@mips.com>