of_batterydata: Add function declarations

Currently, of_batterydata.h is not included by of_batterydata.c
which results in errors with checker tools because of missing
function declarations. Fix it.

Change-Id: Id1d29f508e2b7f0ae79398210cf7f13f9d51f39b
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>