gpio: davinci: get rid of DAVINCI_N_GPIO

Since Davinci GPIO driver is moved to support gpiolib it has to use
ARCH_NR_GPIOS (can be configured using CONFIG_ARCH_NR_GPIO Kconfig
option) configuration instead of any mach/platform specific options.

Hence, replace private DAVINCI_N_GPIO with common ARCH_NR_GPIOS. This is
safe because default value for ARCH_NR_GPIOS=256 and maximum number of
supported GPIOs for Davinci is DAVINCI_N_GPIO=144.

More over, this is one of steps to re-use Davinci GPIO driver by other
mach/platform.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>