dts: arm/arm64: remove DTS 'label' property requirement from gic and timer

The armv8 timer, arm gic, and arm gic-v3-its don't use or need the
devicetree label property.  Update the dts bindings to not require it and
remove setting of the label property in dts files.

Signed-off-by: Kumar Gala <galak@kernel.org>