soc: rockchip: add reboot-mode header

Rockchip platform use a SYSCON mapped register store
the reboot mode magic value for bootloader to use when
system reboot.

Add the shared header describing the values firmware expects
for different boot modes.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>