clk: tegra: add header for common tegra clock IDs

Many clocks are common between several Tegra SoCs. Define an enum to list
them so we can move them to separate files which can be shared between
SoCs. Each SoC specific file will provide an array with the common clocks
which are present on the SoC and their DT binding ID.

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