Implement low power functionality on USB subsystem

This enables remote wakeup on CDC driver, so we can safely call standby() and maintain the Serial prints on resume.

cores/arduino/USB/SAMD21_USBDevice.h

@@ -48,6 +48,7 @@ public:
 
 	inline void runInStandby()   { usb.CTRLA.bit.RUNSTDBY = 1; }
 	inline void noRunInStandby() { usb.CTRLA.bit.RUNSTDBY = 0; }
+	inline void wakeupHost()     { usb.CTRLB.bit.UPRSM = 1; }
 
 	// USB speed
 	inline void setFullSpeed()       { usb.CTRLB.bit.SPDCONF = USB_DEVICE_CTRLB_SPDCONF_FS_Val;   }

cores/arduino/USB/USBAPI.h

@@ -65,6 +65,8 @@ public:
 	bool configured();
 	bool connected();
 
+	void standby();
+
 	// Setup API
 	bool handleClassInterfaceSetup(USBSetup &setup);
 	bool handleStandardSetup(USBSetup &setup);

cores/arduino/USB/USBCore.cpp

@@ -250,6 +250,11 @@ bool USBDeviceClass::sendDescriptor(USBSetup &setup)
 	return true;
 }
 
+void USBDeviceClass::standby() {
+	usbd.noRunInStandby();
+}
+
+
 void USBDeviceClass::handleEndpoint(uint8_t ep)
 {
 #if defined(CDC_ENABLED)

cores/arduino/USB/USBCore.h

@@ -258,7 +258,7 @@ _Pragma("pack()")
  *    idVendor, idProduct, bcdDevice, iManufacturer, iProduct, iSerialNumber, bNumConfigurations */
 
 #define D_CONFIG(_totalLength,_interfaces) \
-	{ 9, 2, _totalLength,_interfaces, 1, 0, USB_CONFIG_BUS_POWERED, USB_CONFIG_POWER_MA(500) }
+	{ 9, 2, _totalLength,_interfaces, 1, 0, USB_CONFIG_BUS_POWERED | USB_CONFIG_REMOTE_WAKEUP, USB_CONFIG_POWER_MA(500) }
 /* Table 9-10. Standard Configuration Descriptor
  * bLength, bDescriptorType, wTotalLength, bNumInterfaces, bConfigurationValue, iConfiguration
  * bmAttributes, bMaxPower */