ARM: shmobile: armadillo800eva: enable rw rootfs mount

armadillo800eva default boot loader is "hermit",
and it's tag->u.core.flags has flag when kernel boots.
Because of this, ${LINUX}/arch/arm/kernel/setup.c :: parse_tag_core()
didn't remove MS_RDONLY flag from root_mountflags.
Thus, the rootfs is mounted as "readonly".
This patch adds "rw" kernel parameter,
and enable read/write mounts for rootfs

Cc: Masahiro Nakai <nakai@atmark-techno.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms@verge.net.au>