Add sleep & wakeup (thanks rocketscream)

SerialFlash.h

 	static bool begin();
 	static uint32_t capacity(const uint8_t *id);
 	static uint32_t blockSize();
+	static void sleep();
+	static void wakeup();
 	static void readID(uint8_t *buf);
 	static void read(uint32_t addr, void *buf, uint32_t len);
 	static bool ready();

SerialFlashChip.cpp

 	return true;
 }
 
+void SerialFlashChip::sleep()
+{
+	if (busy) wait();
+	SPI.beginTransaction(SPICONFIG);
+	CSASSERT();
+	SPI.transfer(0xB9); // Deep power down command
+	CSRELEASE();
+}
+
+void SerialFlashChip::wakeup()
+{
+	SPI.beginTransaction(SPICONFIG);
+	CSASSERT();
+	SPI.transfer(0xAB); // Wake up from deep power down command
+	CSRELEASE();
+}
+
 void SerialFlashChip::readID(uint8_t *buf)
 {
 	if (busy) wait();