mmc: sh_sdhi: Add 64-bit access to sd_buf support

Renesas SDHI SD/MMC driver has 16-bit width bus access to SD_BUF.
This adds 64-bit width bus access to SD_BUF.

Signed-off-by: Kouei Abe <kouei.abe.cp@renesas.com>
Cc: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>