avr32: function for independently setting up SPI slaves

at32ap_spi_setup_slaves() is exposed to allow the addition of SPI slaves
to a SPI bus that has already been registered.  This is especially
useful for add-on cards, which have their own board setup function.
at32ap_spi_add_device() has been slightly re-organized, but should be
transparent to existing users.

So to add a SPI slave, after a SPI bus has already been registered:

	/* Configure the list of slaves on SPI controller "id" */
	at32_spi_setup_slaves(id,spi_board_info,ARRAY_SIZE(spi_board_info));
 	/* Register the list of slaves */
 	spi_register_board_info(spi_board_info,ARRAY_SIZE(spi_board_info));

Signed-off-by: Peter Ma <pma@mediamatech.com>
[haavard.skinnemoen@atmel.com: trivial whitespace fix]
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>