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>
diff --git a/boot/zephyr/include/io/io.h b/boot/zephyr/include/io/io.h
index 332eefb..145530b 100644
--- a/boot/zephyr/include/io/io.h
+++ b/boot/zephyr/include/io/io.h
@@ -35,6 +35,11 @@
 void io_led_init(void);
 
 /*
+ * Sets value of the configured LED.
+ */
+void io_led_set(int value);
+
+/*
  * Checks if GPIO is set in the required way to remain in serial recovery mode
  *
  * @retval	false for normal boot, true for serial recovery boot