gpio: omap: add a GPIO_OMAP option instead of using ARCH_OMAP

The ARCH_OMAP config option was used to built the GPIO OMAP
driver but this is not consistent with the rest of the GPIO
drivers that have their own Kconfig option.

Also, this make it harder to add dependencies or reverse
dependencies (i.e: select) since that would mean touching the
sub-arch config option.

So is better to add a boolean Kconfig option for this driver
that defaults to true if ARCH_OMAP is enabled.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>