sh: add calling mmc_initialize in board.c

Some SH have MMC controller. So, if we need it, we have to call
the mmc_initialize().

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>