gpio: always enable GPIO_OMAP on ARCH_OMAP

Commit 4df42de9d3e "gpio: omap: add a GPIO_OMAP option instead of using
ARCH_OMAP" made it possible to build OMAP kernels without the GPIO driver,
which at least on OMAP2 and OMAP3 causes build errors because of functions
used by the platform power management code:

arch/arm/mach-omap2/built-in.o: In function `omap_sram_idle':
arch/arm/mach-omap2/pm24xx.c:129: undefined reference to `omap2_gpio_prepare_for_idle'
arch/arm/mach-omap2/pm24xx.c:129: undefined reference to `omap2_gpio_resume_after_idle'

We presumably always want the GPIO driver on OMAP, so this adds a slightly
broader dependency and only allows disabling the driver only when no
OMAP2PLUS platform is selected.

However, it seems entirely reasonable to include the driver in build tests
on other platforms, so we should also allow building it for COMPILE_TEST
builds and select the required GENERIC_IRQ_CHIP that may not already be
enabled on other platforms.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>