arm64: dts: broadcom: bcm4908: add DT for Netgear RAXE500

It's a home router based on BCM4908 SoC. It has: 1 GiB of RAM, 512 MiB
NAND flash, 6 Ethernet ports and 3 x BCM43684 (WiFi). One of Ethernet
ports is "2.5 G Multi-Gig port" that isn't described yet (it isn't known
how it's wired up).

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