target-arm: Add missing MAIR_EL3 and TPIDR_EL3 registers

Add the AArch64 registers MAIR_EL3 and TPIDR_EL3, which are the only
two which we had implemented the 32-bit Secure equivalents of but
not the 64-bit Secure versions.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 1438281398-18746-2-git-send-email-peter.maydell@linaro.org