ARM: mstar: Add reboot support

MStar v7 SoCs support reset by writing a magic value to a register
in the "pmsleep" area.

This adds a node for using the syscon reboot driver to trigger a reset.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>