sf: stmicro: support JEDEC standard two-byte signature

There are more than the M25Pxx serial flashs that can be
used with the stmicro driver, for example: the M25PXxx or
N25Qxx serie. All these chips have burned in the original
stmicro manufacture id 0x20 together with a standard
two-byte signature.

In preperation to support all these chips the stmicro driver
have to decode the full two-byte signature.

Signed-off-by: Stephan Linz <linz@li-pro.net>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>