platform/x86: intel_pmc_ipc: Add read64 API

Add intel_pmc_gcr_read64() API for reading from 64-bit GCR registers.
This API will be called from intel_telemetry. Update description of
intel_pmc_gcr_read().

Signed-off-by: Souvik Kumar Chakravarty <souvik.k.chakravarty@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>