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/arch/arm/mach-socfpga/Kconfig b/arch/arm/mach-socfpga/Kconfig
index 1c10ffb..5be9eaa 100644
--- a/arch/arm/mach-socfpga/Kconfig
+++ b/arch/arm/mach-socfpga/Kconfig
@@ -18,6 +18,9 @@
 config SPL_SERIAL_SUPPORT
 	default y
 
+config SPL_SPI_FLASH_SUPPORT
+	default y if DM_SPI
+
 config TARGET_SOCFPGA_ARRIA5
 	bool
 	select TARGET_SOCFPGA_GEN5