io_timer: reserve pins & timers on first use

This allows modules to do a first-come-first-serve pin/timer reservation
on bootup.
E.g. camera trigger reserves any of the pins, and then PWM/DShot output
will just use the rest of the available pins.

src/drivers/drv_pwm_trigger.h

@@ -49,8 +49,9 @@ __BEGIN_DECLS
  *
  * @param channel_mask	Bitmask of channels (LSB = channel 0) to enable.
  *			This allows some of the channels to remain configured
- *			as GPIOs or as another function.
- * @return		OK on success.
+ *			as GPIOs or as another function. Already used channels/timers
+ *			will not be configured as PWM.
+ * @return <0 on error, the initialized channels mask.
  */
 __EXPORT extern int	up_pwm_trigger_init(uint32_t channel_mask);
 
@@ -78,4 +79,4 @@ __EXPORT extern void	up_pwm_trigger_arm(bool armed);
  */
 __EXPORT extern int	up_pwm_trigger_set(unsigned channel, uint16_t value);
 
-__END_DECLS
+__END_DECLS