[ARM] pxa: separate GPIOs and their mode definitions to pxa2xx-gpio.h

two reasons:
1. GPIO namings and their mode definitions are conceptually not part
   of the PXA register definitions

2. this is actually a temporary move in the transition of PXA2xx to
   use MFP-alike APIs (as what PXA3xx is now doing), so that legacy
   code will still work and new code can be added in step by step

Signed-off-by: eric miao <eric.miao@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>