gpio: pxa: convert to one gpiochip

The pxa gpio IP is provided by one chip, which holds multiple banks.

Another reason the driver should register only one gpiochip instead of
multiple gpiochips (ie. 1 per each bank) is that for pincontrol and
devicetree integration (think gpio-ranges), it's impossible to have the
contiguous pin range 0..127 mapped to gpios 0..127.

This patch, amongst other thinks, paves the path to loosen the bond with
the global structure variable pxa_gpio_chip.
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>