power: fg-util: add median filter for circular buffer

A median filter is useful for filtering out outliers. Add it.

Change-Id: I21f97a870c262e5fb3d33b8250a2bf074f519b58
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>