[ARM] 4559/1: pxa: make PXA_LAST_GPIO a run-time variable

This definition produces processor specific code in generic function
pxa_gpio_mode(), thus creating inconsistencies for support of pxa25x
and pxa27x in a single zImage.

As David Brownell suggests, make it a run-time variable and initialize
at run-time according to the number of GPIOs on the processor. For now
the initialization happens in pxa_init_irq_gpio(),  since there is
already a parameter for that, besides, this is and MUST be earlier
than any subsequent calls to pxa_gpio_mode().

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