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>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>