ARM: BCM5301X: Add Broadcom's bus-axi to the DTS file

This has been successfully tested on Netgear R6250 and two other
development (unnamed) devices, all of them BCM4708 based.
We also got a possitive feedback from R7000 (BCM4709) tester.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>