[AVR32] Use per-controller spi_board_info structures

Set up one spi_board_info array per controller and pass this to
at32_add_device_spi so that it can set up any GPIO pins for chip
selects based on this information.

Extracted from a patch by David Brownell and adapted slightly.

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