bgmac: make bgmac depend on bcm47xx

bgmac uses bcm47xx_nvram.h which is only available when BCM47XX was
selected. Earlier BCMA_HOST_SOC depended on BCM47XX so this was not
build on any other archs, but that changed. We should modify this
driver to get access to the nvram or the variables through platform
data.

This fixes a build problem in linux-next reported by Stephen Rothwell:

drivers/net/ethernet/broadcom/bgmac.c:19:27: fatal error: bcm47xx_nvram.h: No such file or directory
 #include <bcm47xx_nvram.h>
                           ^

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: John W. Linville <linville@tuxdriver.com>