Add mutex to protect ras shared memory.

v2:
  Add TA_RAS_COMMAND__TRIGGER_ERROR command call
  status check.
Signed-off-by: YiPeng Chai <YiPeng.Chai@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>