ArmPkg/Library: remove FFA_ERROR() return for FFA_SEND_DIRECT_MSG_REQ(v2)

There is a known issue [0] where the SPMC repeatedly re-enters
Standalone MM with FFA_ERROR after Standalone MM returns FFA_ERROR
because it failed to locate an MMI handler for an MM communication
request.

Although the FF-A specification does not currently clarify this
behavior, it is expected to be updated. Once Standalone MM is entered
through FFA_MSG_SEND_DIRECT_REQ or FFA_MSG_SEND_DIRECT_REQ2, it must
return through FFA_MSG_SEND_DIRECT_RESP or FFA_MSG_SEND_DIRECT_RESP2,
respectively.

Therefore, do not return FFA_ERROR after Standalone MM has been entered
through a direct request.

Standalone MM handles two types of services received through direct
requests: ServiceTypeMmCommunication and ServiceTypeMisc.

For ServiceTypeMmCommunication, protocol errors are returned through
the communication buffer, so the handler itself should return
EFI_SUCCESS. However, failures may occur outside the handler, such as
when no matching MMI handler can be found. In these cases, construct the
direct response as follows:

  - FFA_MSG_SEND_DIRECT_RESP:
      x0 = ARM_FID_FFA_MSG_SEND_DIRECT_RESP
      x1 = Source/destination partition IDs
      x2 = 0x00
      x3 = ARM_FID_SPM_MM_SP_EVENT_COMPLETE
      x4 = ARM_SPM_MM_RET_*

  - FFA_MSG_SEND_DIRECT_RESP2:
      x0 = ARM_FID_FFA_MSG_SEND_DIRECT_RESP2
      x1 = Source/destination partition IDs
      x2 = 0x00 (SBZ)
      x3 = 0x00 (SBZ)
      x4 = ARM_FID_SPM_MM_SP_EVENT_COMPLETE
      x5 = ARM_SPM_MM_RET_*

Although this convention is not formally documented, it has been used
since edk2 commit 388dfe02fb9a ("StandaloneMmPkg: Add option to use...").

For ServiceTypeMisc, the handler returns the complete response,
including any protocol-specific error code and arguments, directly
through FfaArgs. The handler is therefore responsible for constructing
FfaArgs and should return EFI_SUCCESS.

All ServiceTypeMisc handlers are registered during Standalone MM
initialization, and initialization fails if any handler cannot be
registered. A failure returned by a ServiceTypeMisc handler is therefore
unexpected.

There is no generic way to construct FfaArgs for a direct response in
this situation. Call CpuDeadLoop() instead of returning an invalid
response.

Closes: https://github.com/tianocore/edk2/issues/12646#issuecomment-5523596676 [0]
Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>

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