arm64: tegra: Add Tegra186 support

This adds the initial support of Tegra186 SoC. It provides enough to
enable the serial console and boot from an initial ramdisk.

Signed-off-by: Joseph Lo <josephl@nvidia.com>
[treding@nvidia.com: remove leading 0 from unit-addresses]
[treding@nvidia.com: remove unused nvidia,bpmp property]
Signed-off-by: Thierry Reding <treding@nvidia.com>