gpio/omap: auto request GPIO as input if used as IRQ via DT

When an OMAP GPIO is used as an IRQ line, a call to gpio_request()
has to be made to initialize the OMAP GPIO bank before a driver
request the IRQ. Otherwise the call to request_irq() fails.

Drives should not be aware of this neither care wether an IRQ line
is a GPIO or not. They should just request the IRQ and this has to
be handled by the irq_chip driver.

With the current OMAP GPIO DT binding, if we define:

    gpio6: gpio@49058000 {
    	   compatible = "ti,omap3-gpio";
	   reg = <0x49058000 0x200>;
	   interrupts = <34>;
	   ti,hwmods = "gpio6";
	   gpio-controller;
	   #gpio-cells = <2>;
	   interrupt-controller;
	   #interrupt-cells = <2>;
    };

	   interrupt-parent = <&gpio6>;
           interrupts = <16 8>;

The GPIO is correctly mapped as an IRQ but a call to gpio_request()
is never made. Since a call to the custom IRQ domain .map function
handler is made for each GPIO used as an IRQ, the GPIO can be setup
and configured as input there automatically.

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
  - Add a comment that this is just a temporary solution until and
    that it has to be removed once is handled by the IRQ core.

Changes since v2:
 - Only make the call to gpio_request_one() conditional in the DT
   case as suggested by Grant Likely.

Changes since v1:
  - Split the irq domain mapping function handler and the GPIO
    request in two different patches.

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>