mmc: sh_sdhi: Add SDHI support

R-Car Gen3 series have four SD card interfaces (SDHI0 to SDHI3),
two of which can also be used as MMC interfaces (SDHI2 and SDHI3).
This adds High-speed mode SD clock frequency between 25MHz and 50MHz,
8bit/4bit bus width, high capacity and low voltage device support.

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>