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-zynq/Kconfig b/arch/arm/mach-zynq/Kconfig
index 007cad4..9dcd841 100644
--- a/arch/arm/mach-zynq/Kconfig
+++ b/arch/arm/mach-zynq/Kconfig
@@ -6,6 +6,9 @@
 config SPL_LIBCOMMON_SUPPORT
 	default y
 
+config SPL_LIBDISK_SUPPORT
+	default y
+
 config SYS_BOARD
 	default "zynq"