soc/tegra: pmc: Use register definitions instead of magic values

Use register definitions for the main SoC reset operation instead of
hard-coding magic values. Note that the PMC_RST_STATUS register isn't
actually accessed, but since it is mentioned in a comment the
definitions are added for completeness.

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