ARM: shmobile: silk: add eMMC DT support

Define the SILK board dependent part of the MMCIF device node (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 the original patch by Vladimir Barinov
<vladimir.barinov@cogentembedded.com>.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>