mips: dts: realtek: Add gpio block

mips: dts: realtek: Add gpio block

The RTL9300 has a block of GPIOs included in the SoC. Add these to the
devicetree.

This is taken from openwrt[1] the differences are removing the
unnecessary second cell from the interrupt and removing the -controller
from the node name to conform to the dtschema.

[1] - https://git.openwrt.org/?p=openwrt/openwrt.git;a=blob;f=target/linux/realtek/dts/rtl930x.dtsi

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