mips: dts: realtek: Add syscon-reboot node

The board level reset on systems using the RTL9302 can be driven via the
switch. Use a syscon-reboot node to represent this.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>