ARM: dts: BCM5301X: Add DT for Luxul XWR-1200

Luxul XWR-1200 in a dual-band router based on BCM47081. It uses serial
flash (for bootloader and NVRAM) and NAND flash (for firmware).

Signed-off-by: Dan Haab <dhaab@luxul.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>