mfd: wcd9xxx-core: add debugfs entry to dump slimslave registers

Currently, there is a debugfs entry defined for peeking one register
at a time. Add another debugfs entry to dump all SLIMBUS slave
register.

Change-Id: Iddf056d68afd3699d9d8d8eef83bd98d03e61fac
Signed-off-by: Patrick Lai <plai@codeaurora.org>
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>