gpio: omap: Fix regression for MPUIO interrupts

At some point with all the GPIO clean-up we've broken the
MPUIO interrupts. Those are just a little bit different from
the GPIO interrupts, so we can fix it up just by setting
different irqchip functions for it. And then we can just
remove all old code trying to do the same.

Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Javier Martinez Canillas <javier@dowhile0.org>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@linaro.org>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>