bcma: add SOC bus

This patch adds support for using bcma on a Broadcom SoC as the system
bus. An SoC like the bcm4716 could register this bus and use it to
searches for the bcma cores and register the devices on this bus.

BCMA_HOSTTYPE_NONE was intended for SoCs at first but BCMA_HOSTTYPE_SOC
is a better name.

Acked-by: Rafał Miłecki <zajec5@gmail.com>
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>