Convert CONFIG_SPL_NAND_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/mach-socfpga/Kconfig b/arch/arm/mach-socfpga/Kconfig
index 6aeabdb..87402e1 100644
--- a/arch/arm/mach-socfpga/Kconfig
+++ b/arch/arm/mach-socfpga/Kconfig
@@ -12,6 +12,9 @@
 config SPL_MMC_SUPPORT
 	default y if DM_MMC
 
+config SPL_NAND_SUPPORT
+	default y if SPL_NAND_DENALI
+
 config TARGET_SOCFPGA_ARRIA5
 	bool
 	select TARGET_SOCFPGA_GEN5