MIPS: BCM47XX: provide sprom to bcma bus

On SoCs the sprom is often stored in nvram in the flashchip. This patch
registers a sprom fallback callback handler in bcma and provides the
sprom needed for this device.

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