arm: Don't consult the TLB test iface for functional translations

arm: Don't consult the TLB test iface for functional translations

Don't consult the TLB test interface for PA's returned by functional
translations by the AT instruction. We implement this by chaning the
ISA code to synthesize 0-length functional reads for the TLB lookup.
The TLB then bypasses the final PA check in the tester if the size is
zero.

Change-Id: I2487b7f829cea88c37e229e9fc7a4543aced961b
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>