TSPD: Require NS preemption along with EL3 exception handling

At present, the build option TSP_NS_INTR_ASYNC_PREEMPT controls how
Non-secure interrupt affects TSPs execution. When TSP is executing:

  1. When TSP_NS_INTR_ASYNC_PREEMPT=0, Non-secure interrupts are received
     at the TSP's exception vector, and TSP voluntarily preempts itself.

  2. When TSP_NS_INTR_ASYNC_PREEMPT=1, Non-secure interrupts causes a
     trap to EL3, which preempts TSP execution.

When EL3 exception handling is in place (i.e.,
EL3_EXCEPTION_HANDLING=1), FIQs are always trapped to EL3. On a system
with GICv3, pending NS interrupts while TSP is executing will be
signalled as FIQ (which traps to EL3). This situation necessitates the
same treatment applied to case (2) above.

Therefore, when EL3 exception handling is in place, additionally
require that TSP_NS_INTR_ASYNC_PREEMPT is set to one 1.

Strictly speaking, this is not required on a system with GICv2, but the
same model is uniformly followed regardless, for simplicity.

Relevant documentation updated.

Change-Id: I928a8ed081fb0ac96e8b1dfe9375c98384da1ccd
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>