ARM: dts: rza2mevb: Add SDHI support

The RZ/A2M EVB supports 2 SD card slots. A micro SD slot on the CPU
board, and a full SD card slot on the sub board.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>