[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>