bcma: read out some additional sprom attributes

This code is copied from the ssb sprom read code. These attributes are
partly used by b43 and brcmsmac and should also be read out on bcma
based devices.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Tested-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>