ARM: mach-shmobile: ap4evb: Mark NOR boot loader partitions read-only.

This makes the loader and bootenv partitions read-only under MTD for the
on-board NOR flash.

Signed-off-by: Yusuke Goda <yusuke.goda.sx@renesas.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>