[MTD] [NOR] Add support for flash chips with ID in bank other than 0

According to JEDEC "Standard Manufacturer's Identification Code"
(http://www.jedec.org/download/search/jep106W.pdf)
several first banks of NOR flash can contain 0x7f instead of actual ID.
This patch adds support for reading manufacturer ID from banks other than 0.

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>