ARM: rmobile: Enable CONFIG_ARCH_FIXUP_FDT_MEMORY on Gen3

Enable CONFIG_ARCH_FIXUP_FDT_MEMORY to let U-Boot patch the
Linux DT memory entries.

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