It is ILLEGAL to set STE.S1STALLD to 1 if stage 1 is enabled and
either the stall or terminate models are not supported.

This patch fixes the STALLD check and ensures that we don't set STALLD
in the STE when it is not supported.
Signed-off-by: Prem Mallappa <pmallapp@broadcom.com>
[will: consistently use IDR0_STALL_MODEL_* prefix]
Signed-off-by: Will Deacon <will.deacon@arm.com>