gpio: davinci: don't create irq_domain in case of unbanked irqs

The system may crash if:
- there are more than 1 banks
- unbanked irqs are enabled
- someone will call gpio_to_irq() for GPIO from bank2 or above

Hence, fix it by not creating irq_domain if unbanked irqs are enabled
and correct gpio_to_irq_banked() to handle this properly.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Alexandre Courbot <gnurou@gmail.com>
Cc: Sekhar Nori <nsekhar@ti.com>

Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>