gpio: omap: convert gpio irq functions to use GPIO offset

Convert GPIO IRQ functions to use GPIO offset instead of system
GPIO numbers. This allows to drop unneeded conversations between
system GPIO <-> GPIO offset which are done in many places and
many times.
It is safe to do now because:
- gpiolib always passes GPIO offset to GPIO controller
- OMAP GPIO driver converted to use IRQ domain, so
  struct irq_data->hwirq contains GPIO offset

This is preparation step before removing:
 #define GPIO_INDEX(bank, gpio)
 #define GPIO_BIT(bank, gpio)
 int omap_irq_to_gpio()

Tested-by: Tony Lindgren <tony@atomide.com>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Acked-by: Javier Martinez Canillas <javier@dowhile0.org>
Signed-off-by: Grygorii Strashko <grygorii.strashko@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>