soc/tegra: pmc: Use consistent naming for PM domains

The various error messages refer to the PM domains as "power domain",
"genpd" and "PM domain". That's confusing, so convert all error messages
to use the most prominent: "PM domain".

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