Documentation: power: add qpnp-charger documentation

Add the qpnp-charger documentation to supplement
the existing devicetree bindings.

Also, define the power supply framework properties
used throughout the qpnp-charger driver and how they
are impacted through other supplicants and supplier
drivers.

Change-Id: I7a772f25674639a2b73758ab10b2ff2f9a553563
Signed-off-by: David Keitel <dkeitel@codeaurora.org>