Rather than rely on explicit initialization order called from SoC setup
code, use a plain initcall and rely on initcall ordering to take care of
dependencies.

This driver exposes some functionality (querying the chip ID) needed at
very early stages of the boot process. An early initcall is good enough
provided that some of the dependencies are deferred to later stages. To
make sure any abuses are easily caught, output a warning message if the
chip ID is queried while it can't be read yet.
Signed-off-by: Thierry Reding <treding@nvidia.com>