ARM: dts: BCM5301X: Add missing memory "device_type" for Luxul XWC-2000

This property is needed since commit abe60a3a ("ARM: dts: Kill off
skeleton{64}.dtsi"). Without it booting silently hangs at:
[    0.000000] Memory policy: Data cache writealloc

Fixes: 984829e2 ("ARM: dts: BCM5301X: Add DT for Luxul XWC-2000")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>