arm64: dts: broadcom: bcm4908: add BCM4906 Netgear R8000P DTS files

Netgear R8000P is home router based on BCM4906 that is a cheaper variant
of BCM4908 (e.g. 2 cores instead of 4).

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