mmc: sh_sdhi: Fix Kconfig entry

The Kconfig entry depends on RMOBILE, but this was renamed
to ARCH_RMOBILE in commit 1cc95f6e (ARM: Rmobile: Rename
CONFIG_RMOBILE to CONFIG_ARCH_RMOBILE) . Fix this omission.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.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>