ARM: pxa: fix gpio_to_chip() clash with gpiolib namespace

The PXA platform code has a static inline helper called
gpio_to_chip which clashes with the gpiolib namespace if we
try to expose the function with the same name from gpiolib,
and it's still confusing even if we don't do that. So rename
it to gpio_to_pxachip().

Reported-by: H Hartley Sweeten <hartleys@visionengravers.com>
Cc: Eric Miao <eric.miao@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>