[ARM] pxa: Avoid pxa_gpio_mode() in gpio_direction_{in,out}put()

pxa_gpio_mode() is a universal call that fiddles with the GAFR
(gpio alternate function register.)  GAFR does not exist on PXA3
CPUs, but instead the alternate functions are controlled via the
MFP support code.

Platforms are expected to configure the MFP according to their
needs in their platform support code rather than drivers.  We
extend this idea to the GAFR, and make the gpio_direction_*()
functions purely operate on the GPIO level.

This means platform support code is entirely responsible for
configuring the GPIOs alternate functions on all PXA CPU types.

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