ARM: dts: Enable SRAB switch and GMACs on 5301x DTS

Add the Switch Register Access Block which is a special piece of
hardware allowing us to perform indirect read/writes towards the
integrated BCM5301X Ethernet switch.

We also add the 4 Gigabit MAC Device Tree nodes within the brcm,bus-axi
bus node to get proper binding between the BCMA instantiated core and
the Device Tree nodes. We will need that to be able to reference
Ethernet Device Tree nodes in a future patch adding the switch ports
layout.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>