[AVR32] /proc/interrupts display

The /proc/interrupts file should also display the irq_chip associated
with each irq ... e.g. INTC, EIM, GPIO.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>