power: qpnp-linear-charger: Add debugfs for devicetree parameters

Add the debugfs support for listing the configurable devicetree parameters.
Also print these parameters with pr_debug during probe.

Change-Id: I88641bcda09b17aa76f81d74af57325693dddd5d
Signed-off-by: Jie Cheng <rockiec@codeaurora.org>