target-arm: Add arm_ccnt_enabled function

Include a helper function to determine if the CCNT counter
is enabled.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: e1a64f17a756e06c8bda8238ad4826d705049f7a.1409025949.git.peter.crosthwaite@xilinx.com
[ PC changes
  * Remove EL based checks
]
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
