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>