arm/tegra: convert tegra20 to GIC devicetree binding

Convert tegra20 IRQ intialization to the GIC devicetree binding. Modify the
interrupt definitions in the dts files according to
Documentation/devicetree/bindings/arm/gic.txt

v3 (swarren):
* Moved of_irq_init() call into board-dt.c to avoid ifdef'ing it.
  - Even with a dummy replacement if !CONFIG_OF, the reference from
    tegra_dt_irq_match[] to gic_of_init() would still have to be ifdef'd
  - It's plausible that tegra_dt_irq_match[] may need to contain more
    entries in the future, and defining what they are seems more suitable
    for board-dt.c than irq.c
v2 (swarren):
* Removed some stale GIC init code from board-dt.c
* Undid some accidental 0x -> 0x0 search/replace.

Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
[olof: added include of <asm/hardware/gic.h> for compile to pass]
Signed-off-by: Olof Johansson <olof@lixom.net>