x86 tsc: Add the Intel Denverton Processor to native_calibrate_tsc()

The Intel Denverton microserver uses a 25 MHz TSC crystal,
so we can derive its exact * TSC frequency
using CPUID and some arithmetic, eg.

TSC: 1800 MHz (25000000 Hz * 216 / 3 / 1000000)

* 'exact' is only as good as the crystal, which should be +/- 20ppm

Signed-off-by: Len Brown <len.brown@intel.com>