ULP: Add example of using GPIO to wakeup the ULP-RISCV processor

examples/system/ulp_riscv/gpio_interrupt/main/ulp/main.c

@@ -0,0 +1,31 @@
+/*
+ * SPDX-FileCopyrightText: 2022 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+/* ULP-RISC-V example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+
+   This code runs on ULP-RISC-V  coprocessor
+*/
+
+#include "ulp_riscv.h"
+#include "ulp_riscv_utils.h"
+
+int main (void)
+{
+    ulp_riscv_wakeup_main_processor();
+
+    /* Wakeup interrupt is a level interrupt, wait 1 sec to
+       allow user to release button to avoid waking up the ULP multiple times */
+    ulp_riscv_delay_cycles(1000*1000 * ULP_RISCV_CYCLES_PER_US);
+    ulp_riscv_gpio_wakeup_clear();
+
+    /* ulp_riscv_halt() is called automatically when main exits */
+    return 0;
+}