ARM: dts: rockchip: add syscon-reboot-mode DT node

Rockchip platform use a SYSCON mapped register store
the reboot mode magic value for bootloader to use when
system reboot. So add syscon-reboot-mode driver DT node
for rk3xxx/rk3036/rk3288 based platform

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>