ARM: shmobile: silk: add SDHI1 DT support

Define the SILK board dependent part of the SDHI1 (connected to micro-SD
slot) device nodes along with the necessary voltage regulators.

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>