devfreq: devfreq spdm: Add debugfs support

Add debugfs support for the devfreq spdm driver.
The parameters used for determing the SPDM port
threshold value can be updated via debugfs and sent
to the hypervisor to support fine tuning SPDM
performance.

Change-Id: I6f85deacd7d463d90f512f5de18b7e2140c9f492
Signed-off-by: Dan Sneddon <dsneddon@codeaurora.org>