hw/timer: Introduce ARM A9 Global Timer.

The ARM A9 MPCore has a timer that is global to all cores in the cluster.
The timer is shared but each core has a private independent comparator
and interrupt.

Based on version contributed by Francois LEGAL.

Signed-off-by: François LEGAL <devel@thom.fr.eu.org>
Message-id: 4918e89476b8da916be2964ec41578b50d569a37.1385969450.git.peter.crosthwaite@xilinx.com
[PC changes:
 * New commit message
 * Re-implemented as single timer model
 * Fixed backwards counting issue in polled mode
 * completed VMSD fields
 * macroified magic numbers (and headerified reg definitions)
 * split of as device-model-only patch
 * use bitops for 64 bit register access
 * Fixed auto increment mode to check condition properly
 * general cleanup (names/style etc).
]
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
[PMM:
 * minor typo fixes
 * added missing return after error_setg()
 * dropped setting dc->no_user = 1
]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
