fix(bl31): allow use of EHF with S-EL2 SPMC

fix(bl31): allow use of EHF with S-EL2 SPMC

Currently, when SPMC at S-EL2 is used, we cannot use the RAS framework
to handle Group 0 interrupts. This is required on platforms where first
level of triaging needs to occur at EL3, before forwarding RAS handling
to a secure partition running atop an SPMC (hafnium).
The RAS framework depends on EHF and EHF registers for Group 0
interrupts to be trapped to EL3 when execution is both in secure world
and normal world. However, an FF-A compliant SPMC requires secure
interrupts to be trapped by the SPMC when execution is in S-EL0/S-EL1.
Consequently, the SPMC (hafnium) is incompatible with EHF, since it is
not re-entrant, and a Group 0 interrupt trapped to EL3 when execution is
in secure world, cannot be forwarded to an SP running atop SPMC.
This patch changes EHF to only register for Group 0 interrupts to be
trapped to EL3 when execution is in normal world and also makes it a
valid routing model to do so, when EL3_EXCEPTION_HANDLING is set (when
enabling the RAS framework).

Signed-off-by: Raghu Krishnamurthy <raghu.ncstate@gmail.com>
Change-Id: I72d4cf4d8ecc549a832d1c36055fbe95866747fe