ARM: BCM5301X: Enable UART on Netgear R8000

It was tested by LEDE users, all we need is to adjust clock frequency.
While we're at it create a separated DTS include file to share code with
other BCM4709 devices easier.

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