ARM: Kconfig update to support additional GPIOs in OMAP5

OMAP5 has 8 GPIO banks so that there are 32x8 = 256 GPIOs.
In order for the gpiolib to detect and initialize these
additional GPIOs and other TWL GPIOs, ARCH_NR_GPIO is set
to 512 instead of present 256.

Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Cousson, Benoit <b-cousson@ti.com>
Reported-by: Govindraj.R <govindraj.raja@ti.com>
Tested-by: Govindraj.R <govindraj.raja@ti.com>
Signed-off-by: Tarun Kanti DebBarma <tarun.kanti@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>