arm: tcg: Adhere to SMCCC 1.3 section 5.2

arm: tcg: Adhere to SMCCC 1.3 section 5.2

The SMCCC 1.3 spec section 5.2 says

  The Unknown SMC Function Identifier is a sign-extended value of (-1)
  that is returned in the R0, W0 or X0 registers. An implementation must
  return this error code when it receives:

    * An SMC or HVC call with an unknown Function Identifier
    * An SMC or HVC call for a removed Function Identifier
    * An SMC64/HVC64 call from AArch32 state

To comply with these statements, let's always return -1 when we encounter
an unknown HVC or SMC call.

Signed-off-by: Alexander Graf <agraf@csgraf.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>