arm_mptimer: Convert to use ptimer

arm_mptimer: Convert to use ptimer

Current ARM MPTimer implementation uses QEMUTimer for the actual timer,
this implementation isn't complete and mostly tries to duplicate of what
generic ptimer is already doing fine.

Conversion to ptimer brings the following benefits and fixes:
	- Simple timer pausing implementation
	- Fixes counter value preservation after stopping the timer
	- Properly handles prescaler != 0 / counter = 0 / load = 0 cases
	- Code simplification and reduction

Bump VMSD to version 3, since VMState is changed and is not compatible
with the previous implementation.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: 37f378c33bb5a28d5cd71167a6bd5bff5e59cbc3.1475421224.git.digetx@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>