Add a common interface to fdt based SPI drivers.  Each driver is
represented by a table entry in fdt_spi_drivers[].  If there are
multiple SPI drivers in the table, the first driver to return success
from spi_init() will be registered as the SPI driver.
Signed-off-by: NAllen Martin <amartin@nvidia.com>
Signed-off-by: NTom Warren <twarren@nvidia.com>
Reviewed-by: NStephen Warren <swarren@nvidia.com>