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/xilinx_zynqmp_zc1751_xm016_dc2_defconfig b/configs/xilinx_zynqmp_zc1751_xm016_dc2_defconfig
index b9d6c17..da96b96 100644
--- a/configs/xilinx_zynqmp_zc1751_xm016_dc2_defconfig
+++ b/configs/xilinx_zynqmp_zc1751_xm016_dc2_defconfig
@@ -12,6 +12,7 @@
 # CONFIG_SPL_FAT_SUPPORT is not set
 # CONFIG_SPL_LIBDISK_SUPPORT is not set
 # CONFIG_SPL_MMC_SUPPORT is not set
+# CONFIG_SPL_FLASH_SUPPORT is not set
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="ZynqMP> "