Make ARCH_FIXUP_FDT_MEMORY depend on OF_LIBFDT

We need CONFIG_OF_LIBFDT to be able to do fdt fixups, so add that
condition.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/boot/Kconfig b/boot/Kconfig
index 7ef44a2..2dd05e2 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -695,7 +695,7 @@
 
 config ARCH_FIXUP_FDT_MEMORY
 	bool "Enable arch_fixup_memory_banks() call"
-	default y
+	default y if OF_LIBFDT
 	help
 	  Enable FDT memory map syncup before OS boot. This feature can be
 	  used for booting OS with different memory setup where the part of