power: qpnp-charger: add support for external current sensing OVP

On some MSM8x26 platforms, board configurations exist which utilize
a current sensing OVP to allow adaptive input current limiting
to accurately measure the current flow through the external parallel
USB path.

Add support for this external OVP part by allowing an MPP to be
configured as a sense line and exposing the converted current
reading via a module parameter which is accessible to userspace.

CRs-Fixed: 651261
Change-Id: Idb47430476c1c19f58a4836a15841b95187baf77
Signed-off-by: David Keitel <dkeitel@codeaurora.org>