avr32: Use platform_driver_probe for pio platform driver

The probe function of the pio platform driver lives in the init section
and so a pio device that is created after the init section is discarded
probably results in an oops.  Even if this cannot happen, using
platform_driver_probe is cleaner.  (If this can happen and should be
supported the probe function must live in the devinit section instead.)

Signed-off-by: Uwe Kleine-König <ukleinek@strlen.de>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>