Convert CONFIG_SPL_LIBDISK_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 a782daf..ac5af9b 100644
--- a/arch/arm/mach-socfpga/Kconfig
+++ b/arch/arm/mach-socfpga/Kconfig
@@ -3,6 +3,9 @@
 config SPL_LIBCOMMON_SUPPORT
 	default y
 
+config SPL_LIBDISK_SUPPORT
+	default y
+
 config TARGET_SOCFPGA_ARRIA5
 	bool
 	select TARGET_SOCFPGA_GEN5