spmi: Add API to create controller specific debugfs files

Add API function 'spmi_dfs_create_file' for SPMI controller drivers to
create their own implementation specific files under the SPMI debugfs
directory.

Change-Id: Ib4d0557333371a2a29b6dc6ae6ca6a41ebf13c34
Signed-off-by: Kenneth Heitke <kheitke@codeaurora.org>
2 files changed