regulator: fan53555: Add debugfs node to read/write registers

The debugfs node allows read/write access to all the FAN buck
registers. Useful in validating the regulator.

Change-Id: Ie7203d6c1e31a58619d7ff5313f682c961258c37
Signed-off-by: Tirupathi Reddy <tirupath@codeaurora.org>