spi: add common fdt SPI driver interface

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: Allen Martin <amartin@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
diff --git a/drivers/spi/Makefile b/drivers/spi/Makefile
index 78e3d3d..3762d59 100644
--- a/drivers/spi/Makefile
+++ b/drivers/spi/Makefile
@@ -46,6 +46,7 @@
 COBJS-$(CONFIG_SOFT_SPI) += soft_spi.o
 COBJS-$(CONFIG_SH_SPI) += sh_spi.o
 COBJS-$(CONFIG_FSL_ESPI) += fsl_espi.o
+COBJS-$(CONFIG_FDT_SPI) += fdt_spi.o
 COBJS-$(CONFIG_TEGRA20_SFLASH) += tegra20_sflash.o
 COBJS-$(CONFIG_TEGRA20_SLINK) += tegra20_slink.o
 COBJS-$(CONFIG_XILINX_SPI) += xilinx_spi.o