powerpc/85xx: Fix typo in mpc8572ds dts

The localbus node flash had a minor typo for a read-only property.
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

arch/powerpc/boot/dts/mpc8572ds.dts

@@ -89,7 +89,7 @@ nor@0,0 {
 
 			ramdisk@0 {
 				reg = <0x0 0x03000000>;
-				readl-only;
+				read-only;
 			};
 
 			diagnostic@3000000 {