zephyr: io: add 'io_led_set()'

The static declaration of 'led0' was moved to 'io.c' which broke
building with the 'MCUBOOT_INDICATION_LED' enabled:

  mcuboot/boot/zephyr/main.c:380:22: error:
    'led0' undeclared (first use in this function)
      380 |     gpio_pin_set_dt(&led0, 1);
          |                      ^~~~

This adds simple function 'io_led_set()' for changing LED's value.

Fixes: 433b8480 ("zephyr: Move IO functions out of main to separate file")
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>