[AVR32] Implement at32_add_device_cf()

Implement at32_add_device_cf() which will add a platform_device for
the at32_cf driver (not merged yet). Separate out most of the
at32_add_device_ide() code and use it to implement
at32_add_device_cf() as well.

This changes the API in the following ways:
  * The board code must initialize data->cs to the chipselect ID to
    use before calling any of these functions.
  * The board code must use GPIO_PIN_NONE to indicate unused CF pins.

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