power: qpnp-smbcharger: Add single function to read properties from fg

Currently we've separate functions to read different power supply
properties from fg. Since there is a lot of common code in those
functions, consolidate them into single function.

Change-Id: Ib2bf3758e1320f2f01bc12f39ff9376af3d8a7eb
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>