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/l1/stm32l-discovery/button-irq-printf-lowpower/syscfg.h

@@ -44,6 +44,7 @@ extern "C" {
 #define BUTTON_DISCO_USER_isr exti0_isr
 #define BUTTON_DISCO_USER_NVIC NVIC_EXTI0_IRQ
 #define TIMER_BUTTON_PRESS TIM7
+#define TIMER_BUTTON_PRESS_RST RST_TIM7
 
 	struct state_t {
 		bool falling;