Documentation: hyperv: Add overview of clocks and timers

Add documentation topic for clocks and timers when running as a
guest on Hyper-V.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/1657561704-12631-4-git-send-email-mikelley@microsoft.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>