soc/tegra: Register SoC device

Move this code from arch/arm/mach-tegra and make it common among 32-bit
and 64-bit Tegra SoCs. This is slightly complicated by the fact that on
32-bit Tegra, the SoC device is used as the parent for all devices that
are instantiated from device tree.

Signed-off-by: Thierry Reding <treding@nvidia.com>