ARM: pxa: fix invalid mfp pin issue

Failure is reported on hx4700 with kernel v3.3-rc1.

__mfp_validate: GPIO20 is invalid pin
__mfp_validate: GPIO21 is invalid pin
__mfp_validate: GPIO15 is invalid pin
__mfp_validate: GPIO78 is invalid pin
__mfp_validate: GPIO79 is invalid pin
__mfp_validate: GPIO80 is invalid pin
__mfp_validate: GPIO33 is invalid pin
__mfp_validate: GPIO48 is invalid pin
__mfp_validate: GPIO49 is invalid pin
__mfp_validate: GPIO50 is invalid pin

Since pxa_last_gpio is used in mfp-pxa2xx driver. But it's only
updated in pxa-gpio driver that run after mfp-pxa2xx driver.

So update the pxa_last_gpio first in mfp-pxa2xx driver.

Reported-by: Paul Parsons <lost.distance@yahoo.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>