soc/tegra: fuse: Extract tegra_get_platform()

This function extracts the PRE_SI_PLATFORM field from the HIDREV
register and can be used to determine which platform the kernel runs on
(silicon, simulation, ...). Note that while only Tegra194 and later
define this field, it should be safe to call this on prior generations
as well since this field should read as 0, indicating silicon.

Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>