Define the Alt board dependent part of the MMCIF device node.

Like the Silk the board has eMMC chip along with the  necessary voltage
regulator (note that the Vcc/Vccq regulator is dummy -- it's required by
the MMCIF driver but doesn't actually exist on the board).

Based on work for the Silk board by Vladimir Barinov and Sergei Shtylyov.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>