Add APIs for boards with companion chip (eg. Tian)

examples/TianStandby/TianStandby.ino

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+/*
+  TianStandby
+
+  This sketch demonstrates the usage of SAMD chip to furtherly reduce the power usage of Tian
+  board. This method can be applied to any board with companion chips which expose a method 
+  (via direct pin interrupt or via a command) to enter and exit standby.
+  Sleep modes allow a significant drop in the power usage of a board while it does nothing waiting for an event to happen. Battery powered application can take advantage of these modes to enhance battery life significantly.
+
+  In this sketch, the internal RTC of SAMD chip will wake up the processor every 20 seconds.
+  Before going to sleep, the SAMD chip tells the MIPS CPU to standby too.
+  Please note that, if the processor is sleeping, a new sketch can't be uploaded. To overcome this, manually reset the board (usually with a single or double tap to the RESET button)
+
+  This example code is in the public domain.
+*/
+
+#include "ArduinoLowPower.h"
+
+#define MIPS_PIN 32 
+
+void MIPS_PM(bool sleep) {
+  pinMode(MIPS_PIN, OUTPUT);
+  digitalWrite(MIPS_PIN, sleep ? LOW: HIGH);
+}
+
+void setup() {
+  pinMode(LED_BUILTIN, OUTPUT);
+  LowPower.companionLowPowerCallback(MIPS_PM);
+  // Uncomment this function if you wish to attach function dummy when RTC wakes up the chip
+  LowPower.attachInterruptWakeup(RTC_ALARM_WAKEUP, onWakeup, CHANGE);
+}
+
+void loop() {
+  digitalWrite(LED_BUILTIN, HIGH);
+  delay(500);
+  digitalWrite(LED_BUILTIN, LOW);
+  delay(500);
+  // Triggers a 2000 ms sleep (the device will be woken up only by the registered wakeup sources and by internal RTC)
+  // The power consumption of the chip will drop consistently
+  // Send sleep command to MIPS CPU and then go to sleep
+  LowPower.companionSleep();
+  LowPower.sleep(20000);
+}
+
+void onWakeup() {
+  // This function will be called once on device wakeup
+  // You can do some little operations here (like changing variables which will be used in the loop)
+  // Remember to avoid calling delay() and long running functions since this functions executes in interrupt context
+
+  // Wakeup the companion chip, too
+  LowPower.companionWakeup();
+}