arm64: dts: ls1088a: Add reboot nodes

ls1088a has a separate reset register block.  Define it in dts and use
it for reboot.

Signed-off-by: Ashish Kumar <Ashish.Kumar@nxp.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>