drm/i915/tgl: add support for reading the timestamp frequency

There are no changes with respect to GEN11, which Paulo wrote.

This gets rid of the "Missing switch case in read_timestamp_frequency"
message at boot for Tiger Lake.

[ Lucas: BSpec: 10742 and 9024, but there's a mismatch on the values.
  Let's say a glitch in the spec. Tested locally and it works. ]

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190817093902.2171-3-lucas.demarchi@intel.com