mmc: tmio: enable SoC specific DMA buswidth settings

Renesas SDHI which is based on TMIO driver has 2 type SoC. 1st one is
used as SH-Mobile series, and 2nd is R-Car series. R-Car series SoC has
DMA buswidth setting register which enables 32bit access.
This patch adds .dma_buswidth and enables it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>