machine: Rewrite most of the GPIO functionality

Split across device types (nrf, dummy) and use registers directly
instead of the HAL.

src/runtime/runtime_nrf.h

@@ -9,11 +9,3 @@ void uart_send(uint8_t c);
 
 void rtc_init();
 void rtc_sleep(uint32_t ticks);
-
-typedef enum {
-	GPIO_INPUT,
-	GPIO_OUTPUT,
-} gpio_mode_t;
-
-void gpio_cfg(uint32_t pin, gpio_mode_t mode);
-void gpio_set(uint32_t pin, bool high);