target-arm: Add the AArch64 view of the Secure physical timer

On CPUs with EL3, there are two physical timers, one for Secure and one
for Non-secure. Implement this extra timer and the AArch64 registers
which access it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 1437047249-2357-2-git-send-email-peter.maydell@linaro.org