avr32: Add PSIF platform devices

This patch adds the PS/2 interface (PSIF) to the device code, split into
two platform devices, one for each port.

The function for adding the PSIF platform device is also added to the
board header file.

Signed-off-by: Hans-Christian Egtvedt <hcegtvedt@atmel.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>