MIPS: bcm47xx: separate functions finding flash window addr

Also check if parallel flash is present at all before accessing it and
add support for serial flash on BCMA bus.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: http://patchwork.linux-mips.org/patch/4738/


Signed-off-by: John Crispin <blogic@openwrt.org>