bcma: get info about flash type SoC booted from

There is an ongoing work on cleaning MIPS's nvram support so it could be
re-used on other platforms (bcm53xx to say precisely).
This will require a bit of extra logic in bcma this patch implements.

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