input: sensors: add self test interface for sensors class

The self test interface is a generic interface for sensor self test.
Add the interface for user space applications to test sensors. Drivers
using sensors class should try to implement the callback if needed.

Change-Id: I008198534940c1357cfb1c7c4ff9b9b37beffdee
Signed-off-by: Oliver Wang <mengmeng@codeaurora.org>