power: qpnp-qg: Update the function names for QG

Update the QG profile handling function names
by adding a prefix "qg_", this is to differentiate
it from the generic battery profile function
definitions.

While at it, expose the linear_interpolate function
for it to be usable in the QG driver.

Change-Id: I58214402b2d79feb25209f5a1451d3f41edf181a
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>