devfreq: devfreq_spdm: Enhance debugfs entries

Add ability to read filter properties from debugfs and add ability to
enable and disable spdm processing.

Change-Id: I1b94c5c44180df2a39097657ecc6606562736d1a
Signed-off-by: Dan Sneddon <dsneddon@codeaurora.org>
Signed-off-by: Alok Chauhan <alokc@codeaurora.org>