[stm32] simplifly peripheral clock setup

There is now a convenience function in libopencm3 that makes the clock enable for peripherals easier across different STM families:
e.g.
rcc_periph_clock_enable(RCC_USART1);

In some cases it could possibly be simplified even more...
Also here are still a few files that use the old function:
- led_hw.h
- actuators_pwm_arch.c
- spektrum_arch.c