presence: Add new tach sensor implementation

Add a new fan speed (tach) presence implementation for the new
PresenceSensor interface.

This fan tach sensor behaves mostly like the existing one, with
the addition of support for multiple tach sensors for the
single PresenceSensor instance, and implementation of the
PresenceSensor start and stop interfaces.

Change-Id: I535a3e2c78ef0f06f10b6f4cda9a7e2d0e9c11db
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
3 files changed