ARM: dts: wheat: add SDHI0 support

Define the Wheat board dependent  part of  the SDHI0 (connected to the
micro-SD slot)  device node along with the necessary voltage regulator.

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