[Why]
The buffer used when calling psp is a shared buffer. If we have multiple calls
at the same time we can overwrite the buffer.

[How]
Add mutex to guard the shared buffer.
Signed-off-by: NBhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: NNicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>