ssb: check for flash presentence

We can not assume parallel flash is always present, there are boards
with *serial* flash and probably some without flash at all.
Define some bits by the way.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Reviewed-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>