ARM: dts: r8a7794: Correct SDHI register size

r8a7794 SDHI ch0 has SD_DMACR which is located in 0x324.
This patch updates register size

Based on work for the r8a7790 by Kuninori Morimoto.

Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>