bcm47xx: prepare to support different buses

Prepare bcm47xx to support different System buses. Before adding
support for bcma it should be possible to build bcm47xx without the
need of ssb. With this patch bcm47xx does not directly contain a
ssb_bus, but a union contain all the supported system buses. As a SoC
just uses one system bus a union is a good choice.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>