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_defconfig b/configs/am57xx_evm_defconfig
index b75346b..80be599 100644
--- a/configs/am57xx_evm_defconfig
+++ b/configs/am57xx_evm_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_OMAP54XX=y
 CONFIG_TARGET_AM57XX_EVM=y
+CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_SPL_STACK_R_ADDR=0x82000000
 CONFIG_DEFAULT_DEVICE_TREE="am57xx-beagle-x15"
 CONFIG_FIT=y