regmap: create a config option for register writes

Create a config option to allow register writes
using debugfs. Currently this requires source code
changes and re-compile. To facilitate client drivers
that are using regmap framework in debugging, would
need a way accessing registers from debugfs. Creating
config option would make this easy for the client drivers.

Change-Id: I1f5016ecff5d16991235c7691df1f120c0977ec2
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>