added irq code for gumstix

Hi Scott,

We tried to control a brushless motor controller using the Gumstix Overo Air COM's dual mode timers.
Using the original omap3-pwm code, we reconfigured TMAR around 100 times per second. At this high update rate, the PWM does not work as expected, i.e. the PWM line does not go down inbetween two timer periods, causing the motor to go offline for a short period of time. You can verify this using a bash script that writes very fast to one of the PWM devices and then have a look at the PWM output using an oscilloscope.

We think that setting the TMAR register asynchronously to the timer's state might cause the problem. Thus, our suggestion is to write TMAR only when a MATCH interrupt occurs, which is implemented in the attached patch.

Best Regards, Tobias