[Why]
When some tools performing psp mailbox attack, the readback value
of register can be a random value which may break psp.

[How]
Use a psp wptr cache machanism to aovid the change made by attack.

v2: unify change and add detailed reason
Signed-off-by: NVictor Zhao <Victor.Zhao@amd.com>
Signed-off-by: NJingwen Chen <Jingwen.Chen2@amd.com>
Reviewed-by: NMonk Liu <monk.liu@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>