1. PWM driver
Add an API, INT pwmStartTimerPair(const INT nTimerIdentity), to enable
pair channels at the same time.
For instance, calling pwmStartTimerPair(PWM_TIMER0) enables
PWM_TIMER0 and PWM_TIMER1 at the same time. Calling
pwmStartTimerPair(PWM_TIMER2) enables
PWM_TIMER2 and PWM_TIMER3 at the same time.

2. Sample code
Add two samples that use the new API to enable PWM_TIMER0/PWM_TIMER1 and
PWM_TIMER2/PWM_TIMER3 at the same time. The samples includes
PWM_TimerPair(PWM_TIMER0) and PWM_TimerPair(PWM_TIMER2).

Change-Id: Iff9e797d952cc3a86f4789964064277ef5c16b57