[AVR32] Put the chip in "stop" mode when halting the system

Make machine_halt() execute a sleep instruction to put the chip in
"stop" mode when the system is halted. This switches off all clocks
except the 32 kHz oscillator, which is needed for the RTC to keep
ticking.
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>

arch/avr32/kernel/process.c

@@ -38,6 +38,13 @@ void cpu_idle(void)
 
 void machine_halt(void)
 {
+	/*
+	 * Enter Stop mode. The 32 kHz oscillator will keep running so
+	 * the RTC will keep the time properly and the system will
+	 * boot quickly.
+	 */
+	asm volatile("sleep 3\n\t"
+		     "sub pc, -2");
 }
 
 void machine_power_off(void)