ARM: dts: BCM5301X: Add Ethernet MAC address to Luxul XWR-3150

Luxul XWR-3150 stores MAC as NVRAM variable. Add NVMEM cell for it and
reference it in the Ethernet interface node.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>