power: ncp6335d: Add debugfs node to read/write registers

The debugfs node allow read/write access to all the NCP
registers. Useful in validating the regulator.

Change-Id: Ia313afad45efccc6831ffea641a3575ef43dbdd0
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>