Convert CONFIG_SPL_SPI_FLASH_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/am57xx_evm_nodt_defconfig b/configs/am57xx_evm_nodt_defconfig
index 5f6f2e3..25cd7b5 100644
--- a/configs/am57xx_evm_nodt_defconfig
+++ b/configs/am57xx_evm_nodt_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_OMAP54XX=y
 CONFIG_TARGET_AM57XX_EVM=y
+CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_SPL_DMA_SUPPORT=y