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>

drivers/gpio/Kconfig

@@ -243,6 +243,13 @@ config GPIO_OCTEON
 	  Say yes here to support the on-chip GPIO lines on the OCTEON
 	  family of SOCs.
 
+config GPIO_OMAP
+	bool "TI OMAP GPIO support"
+	default y if ARCH_OMAP
+	depends on ARM && ARCH_OMAP
+	help
+	  Say yes here to enable GPIO support for TI OMAP SoCs.
+
 config GPIO_PL061
 	bool "PrimeCell PL061 GPIO support"
 	depends on ARM_AMBA