spi_flash: Add spi_flash_probe_fdt() to locate SPI by FDT node

This allows us to put the SPI flash chip inside the SPI interface node,
with U-Boot finding the correct bus and chip select automatically.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/configs/exynos5250-dt.h b/include/configs/exynos5250-dt.h
index bdefee1..74ba2b6 100644
--- a/include/configs/exynos5250-dt.h
+++ b/include/configs/exynos5250-dt.h
@@ -276,6 +276,7 @@
 #define CONFIG_SF_DEFAULT_MODE		SPI_MODE_0
 #define CONFIG_SF_DEFAULT_SPEED		50000000
 #define EXYNOS5_SPI_NUM_CONTROLLERS	5
+#define CONFIG_OF_SPI
 #endif
 
 #ifdef CONFIG_ENV_IS_IN_SPI_FLASH