gpio: pxa: clamp gpio get value to [0,1]

It would be convenient to normalize the return value for gpio_get.

I have checked mach-mmp / mach-pxa / plat-pxa / plat-orion / mach-orion5x.
It's OK for all of them to change this function to return 0 and 1.

Signed-off-by: Neil Zhang <zhangwm@marvell.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>