bcm4908: initial work on the Broadcom BCM4908 target

bcm4908: initial work on the Broadcom BCM4908 target

BCM4906, BCM4908 and BCM49408 are SoCs with 64 bit ARMv8 B53 CPUs.
Upstream Linux is slowly getting support for that SoCs family so it
makes sense to add target for it.

This prepares initial support for:

1. Asus GT-AC5300
BCM4908 based device (4 CPUs) with 1024 MiB RAM, NAND, 8 LAN ports.

2. Netgear R8000P
BCM4906 based device (2 CPUs) with 512 MiB RAM, NAND, 4 LAN ports.

Flashing info will come later as we learn how to generate proper images.

It isn't usable yet (it only produces a bootable kernel) so "source-only"
is used.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>