target/arm: Permit accesses to ELR_Hyp from Hyp mode via MSR/MRS (banked)

target/arm: Permit accesses to ELR_Hyp from Hyp mode via MSR/MRS (banked)

The MSR (banked) and MRS (banked) instructions allow accesses to ELR_Hyp
from either Monitor or Hyp mode. Our translate time check
was overly strict and only permitted access from Monitor mode.

The runtime check we do in msr_mrs_banked_exc_checks() had the
correct code in it, but never got there because of the earlier
"currmode == tgtmode" check. Special case ELR_Hyp.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Luc Michel <luc.michel@greensocs.com>
Message-id: 20180814124254.5229-9-peter.maydell@linaro.org