dts: riscv: neorv32: add gpio nodes

Add devicetree nodes for the NEORV32 GPIO device.

The GPIO port is 64 bits wide, but Zephyr only supports up to 32 bit
wide GPIO ports. The GPIO device is therefore handled as two Zephyr GPIO
devices with a nexus devicetree node mapping pins 0 to 31 to the device
handling the lower half, and pins 32 to 63 to the device handling the
upper half.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>