bcma: add trivial GBIT MAC COMMON driver

GMAC COMMON core is present on BCM4706 and is used for example to access
board PHYs (PHYs can not be accessed directly using GBIT MAC core).

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>