[mcu_periph] replace gpio_output_[high|low] with gpio_[set|clear]

That means using the already existing gpio_set and gpio_clear from libopencm3 for stm32.
And defining these for the lpc.
Merge pull request #498 from flixr/gpio_setclear

[mcu_periph] replace gpio_output_[high|low] with gpio_[set|clear]