avr32: Add hardware power-down function call

This patch adds in the indirect call to pm_power_off(), as is done in
other architectures (e.g. ARM).

Tested on NGW100, with custom board with GPIO control over main DC
power.
Signed-off-by: Peter Ma <pma@mediamatech.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>

arch/avr32/kernel/process.c

@@ -54,6 +54,8 @@ void machine_halt(void)
 
 void machine_power_off(void)
 {
+	if (pm_power_off)
+		pm_power_off();
 }
 
 void machine_restart(char *cmd)