cpu/a9mpcore: Add Global Timer

cpu/a9mpcore: Add Global Timer

Add the global timer to A9 MPCore.

Signed-off-by: François LEGAL <devel@thom.fr.eu.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: ff92f35f438ac671b57d99d823723dd3e62d2c49.1385969450.git.peter.crosthwaite@xilinx.com
[PC Changes:
 * new commit message
 * split off original version as a separate patch
 * Rebased against new mpcore implementation (with struct embedding)
]
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>