bcma: read SPROM and extract MAC from it

In case of BCMA cards SPROM is located in the ChipCommon core, it is
not mapped as separated host window. So far we have met only SPROMs rev
8.
SPROM layout seems to be the same as for SSB buses, so we decided to
share SPROM struct and some defines.
For now we extract MAC address only, this can be improved of course.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>