tests: gpio_basic_api: add overlay for hifive1_revb testing

Use the Arduino A4 and A5 pins which have no dedicated purpose.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

tests/drivers/gpio/gpio_basic_api/boards/hifive1_revb.overlay

@@ -0,0 +1,13 @@
+/*
+ * Copyright (c) 2019 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/ {
+	resources {
+		compatible = "test,gpio_basic_api";
+		out-gpios = <&gpio0 12 0>; /* Arduino A4 */
+		in-gpios = <&gpio0 13 0>;  /* Arduino A5 */
+	};
+};