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/maxbcm_defconfig b/configs/maxbcm_defconfig
index d8b88b5..d93bc1b 100644
--- a/configs/maxbcm_defconfig
+++ b/configs/maxbcm_defconfig
@@ -5,6 +5,7 @@
 CONFIG_TARGET_MAXBCM=y
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
+CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="armada-xp-maxbcm"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y