[ARM] pxa: enable GPIO receivers after configuring pins

'mach-pxa' platforms currently rely on a bootloader to setup GPIO pins
and clear RDH (to enable inputs).

A kernel loaded by a 'minimal' bootloader, that doesn't touch any pins,
will not function correctly; inputs will remain disabled, even after the
pins are configured. The following change fixes the issue and has been
verified on Gumstix Verdex XL6P and a custom PXA270 platform.

Signed-off-by: Timothy Clacy <tcl@phaseone.com>
Signed-off-by: Eric Miao <eric.miao@marvell.com>