gpio: decouple the IOP GPIO driver from platform

This removes the only dependence between the IOP GPIO
driver and the GENERIC_GPIO header in <mach/gpio.h>
and its common implementation in the <asm/*> namespace
by copying the one constant it is using into the driver
file.

Cc: Lennert Buytenhek <kernel@wantstofly.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Mikael Pettersson <mikpe@it.uu.se>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>