target-arm: Implement missing AMAIR registers

The AMAIR registers are for providing auxiliary implementation
defined memory attributes. We already implemented a RAZ/WI
AMAIR_EL1; add the EL2 and EL3 versions for consistency.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 1438281398-18746-3-git-send-email-peter.maydell@linaro.org