target-arm: Add the HSTR_EL2 register

Add the Hypervisor System Trap Register for EL2.

This register is used early in the Linux boot and without it the kernel
aborts with a "Synchronous Abort" error.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: ea5aae4b10283de4705b864fe9d4bd2eaddaacae.1463174342.git.alistair.francis@xilinx.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>