dt-bindings: arm-gic: Add documentation for Tegra210 AGIC

The Tegra AGIC interrupt controller is compatible with the ARM GIC-400
interrupt controller. Add the compatible string and clock information
for the AGIC to the GIC device-tree binding documentation.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>