spi: Mark spi_register_board_info() __devinit

Some systems have SPI devices located on plugin modules which are
enumerated at runtime as devices. The drivers for these plugin modules
need to register their SPI devices at probe() time so want to be able
to call spi_register_board_info() but that function is currently marked
as __init rather than __devinit so this usage isn't legal. Change the
annotation to __devinit to handle this.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>