arm64: dts: broadcom: bcm4908: add Ethernet MAC addr

On most BCM4908 devices MAC address can be read from the bootloader
binary section containing device settings. Use NVMEM to describe that.

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