arm: rmobile: alt: Add external RAM boot

If CONFIG_RMOBILE_EXTRAM_BOOT is enabled, U-Boot is booted from External RAM.
The default boot address is 0x70000000.

Signed-off-by: Hisashi Nakamura <hisashi.nakamura.ak@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>