gpio/omap: don't create an IRQ mapping for every GPIO on DT

When a GPIO is defined as an interrupt line using Device
Tree, a call to irq_create_of_mapping() is made that calls
irq_create_mapping(). So, is not necessary to do the mapping
for all OMAP GPIO lines and explicitly call irq_create_mapping()
on the driver probe() when booting with Device Tree.

Add a custom IRQ domain .map function handler that will be
called by irq_create_mapping() to map the GPIO lines used as IRQ.
This also allows to execute needed setup code such as configuring
a GPIO as input and enabling the GPIO bank.

Changes since v3:
  - Use bank->chip.of_node instead of_have_populated_dt() to check
    DT or legacy boot as suggested by Jean-Christophe PLAGNIOL-VILLARD

Changes since v2:
  - Unconditionally do the IRQ setup in the .map() function and
    only call irq_create_mapping() in the gpio chip init to avoid
    code duplication as suggested by Grant Likely.

Changes since v1:
  - Split the addition of the .map function handler and the
    automatic gpio request in two different patches.
  - Add GPIO IRQ setup logic to the irq domain mapping function.
  - Only call irq_create_mapping for every GPIO on legacy boot.
  - Only setup a GPIO IRQ on the .map function for DeviceTree boot.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: Enric Balletbo i Serra <eballetbo@gmail.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>