flash: nor: {pic32mx,cfi}: fix register names

flash: nor: {pic32mx,cfi}: fix register names

Commit fd43be07265b5f3cf3146f2bb80c1c2fc0a44fcf introduced a
regression: since the register names were changed from those
traditional for MIPS to common GDB scheme the code that makes use of
them needs to be changed accordingly.

This commit restores pic32mx flash driver functionality.

Change-Id: Id18c739390fae36737a02dc30c363d0444f53b96
Reported-by: Louis Rannou <louson@users.sf.net>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/3206
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>