gpio: omap: convert to use irq_domain_add_simple()

The GPIO OMAP driver supports different OMAP SoC families and
not all of them have the needed support to use the linear IRQ
domain mapping like OMAP1 that use the legacy domain mapping.

But this special check is not necessary since the simple IRQ
domain mapping is able to handle both cases. Having a zero
IRQ offset will be interpreted as a linear domain case while
a non-zero value will be interpreted as a legacy domain case.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
gpio: omap: check gpiochip_add() return value

The gpiochip_add() function can fail if the chip cannot
be registered so the return value has to be checked and
the error propagated in case it happens.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
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>
gpio: omap: convert driver to use gpiolib irqchip

Converts the GPIO OMAP driver to register its chained irq
handler and irqchip using the helpers in the gpiolib core.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
MAINTAINERS: update GPIO OMAP driver entry

I've been maintaining this driver by fixing all issues
found while migrating OMAP2+ towards Device Tree based
booting and keeping it up-to-date by using the latest
infraestructure that is provided by the GPIO subsystem.

It would be nice if people know that I care about this
driver and put me in copy when sending patches for it.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>