clocksource: tegra: move to of_clk_get

The new clockframework introduced DT IDs for each clock. To be able to remove
the device registrations, this driver needs to be updated to use the DT IDs.
Note that the actual removal of the clk_register_clkdev() calls will be done
in a later series.

Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>