avr32: Fix GPIO initcall breakage

Add essential system devices, including GPIO controllers, automatically
at core_initcall time. This ensures that the devices are there when the
PIO driver gets initialized at postcore_initcall, fixing a bug exposed
by commit d6634db8 "avr32: Use
platform_driver_probe for pio platform driver".

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>