Add tick rate API to PAL (#1651)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1651

Add a new PAL method to retrieve the conversion ratio between ticks and nanoseconds. This is needed to support conversion between ticks and fixed units, such as the microseconds returned by the XNNPACK profiling API.

Reviewed By: dbort

Differential Revision: D52915789

fbshipit-source-id: 602ce4a1b7cb6e9edbc4215926e508999d195a30