ARM: rmobile: Drop board_mmc_init

The board_mmc_init() is no longer invoked when DM is used, so move
all the pinmux config into board_init() instead.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>