gpio: tegra: Dynamically allocate IRQ base, and support DT

Enhance the driver to dynamically allocate the base IRQ number, and
create an IRQ domain for itself. The use of an IRQ domain ensures that
any device tree node interrupts properties are correctly parsed.

Describe interrupt-related properties in the device tree binding docs,
and the contents of "child" node interrupts property.

Update tegra*.dtsi to specify the required interrupt-related properties.

Finally, remove the definition of TEGRA_GPIO_TO_IRQ; this macro no longer
gives correct results since the IRQ numbers for GPIOs are dynamically
allocated.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Olof Johansson <olof@lixom.net>