ARM: shmobile: r8a7791: tidyup SDHI register size on DTSI

r8a7791 SDHI ch0/ch1 has SD_DMACR which is located in 0x324.
This patch updates register size

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>