ARM: multi_v7_defconfig: Enable BCM47xx/BCM5301x drivers

Add a bunch of required drivers and subsystems:

- BCMA is the on-chip discoverable bus which registers a bunch of
  peripherals
- Enable the BCM47xx watchdog driver to get working system reboot
- Enable the BCM47xx NVRAM/SPROM drivers to be able to fetch MAC
  addresses and other variables needed for system operation
- Make BGMAC (built-in Ethernet adapter) a module

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