mmc: renesas_sdhi_internal_dmac: Add MMC_CAP2_MERGE_CAPABLE

Since this host controller can merge bigger segments if DMA API
layer cam merge the segments, this patch adds the flag.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>