sensor: add frequency type support

Add Frequency ReadingType and Hz units mapping for sensors exposed under
/sensors/frequency.

Ref to gerrit PR introducing the same in PDI:
https://gerrit.openbmc.org/c/openbmc/phosphor-dbus-interfaces/+/83009

Change-Id: I7b2dc504682863cc3243cb7ccf0dbaf4db1e3e55
Signed-off-by: Vishnunithyasoundhar S <vishnunithyasoundhar.s@intel.com>