timer_reset -> rcc_periph_reset_pulse() replace

timer_reset wasn't fully implemented for all timers on all series, and
was simply a wrapper of the RST bits in the RCC registers anyway.

First part of https://github.com/libopencm3/libopencm3/issues/709

examples/stm32/f4/stm32f4-discovery/timer/timer.c

@@ -81,7 +81,7 @@ static void tim_setup(void)
 	nvic_enable_irq(NVIC_TIM2_IRQ);
 
 	/* Reset TIM2 peripheral. */
-	timer_reset(TIM2);
+	rcc_periph_reset_pulse(RST_TIM2);
 
 	/* Timer global mode:
 	 * - No divider