arm/dt: tegra devicetree support

Everything required to populate NVIDIA Tegra devices from the device
tree.  This patch adds a new DT_MACHINE_DESC() which matches against
a tegra20 device tree.  So far it only registers the on-chip devices,
but it will be refined in follow on patches to configure clocks and
pin IO from the device tree also.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>