flash/nor/at91sam4: fix sam4sa16c flash banks and its gpnvms count

flash/nor/at91sam4: fix sam4sa16c flash banks and its gpnvms count

There was already a github fork that had this fixed, but as we try
to use the latest, non-modified version of all software we use,
I would like to have this fix in the next releases of OpenOCD so
that if people uses $packagemanager, they will not have issues flashing
the last part of the flash of sam4sa16c chips.

Additionally, I've added some more logging related to the flash
bank that was used, and the chip ID that was detected.

Change-Id: I7ea5970105906e4560b727e46222ae9a91e41559
Signed-off-by: Erwin Oegema <blablaechthema@hotmail.com>
Reviewed-on: http://openocd.zylin.com/4599
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins