target-arm: Enable EL2 feature bit on A53 and A57

Enable the ARM_FEATURE_EL2 bit on Cortex-A52 and
Cortex-A57, since this is all now sufficiently implemented
to work with the GICv3. We provide the usual CPU property
to disable it for backwards compatibility with the older
virt boards.

In this commit, we disable the EL2 feature on the
virt and ZynpMP boards, so there is no overall effect.
Another commit will expose a board-level property to
allow the user to enable EL2.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 1483977924-14522-18-git-send-email-peter.maydell@linaro.org
