dm: Fix repeated comment in README

A merge error ended up repeating a similar sentence twice. Fix it.
Signed-off-by: NSimon Glass <sjg@chromium.org>

README

@@ -3849,12 +3849,9 @@ Configuration Settings:
 		The memory will be freed (or in fact just forgotton) when
 		U-Boot relocates itself.
 
-		Pre-relocation malloc() is only supported on sandbox
+		Pre-relocation malloc() is only supported on ARM and sandbox
 		at present but is fairly easy to enable for other archs.
 
-		Pre-relocation malloc() is only supported on ARM at present
-		but is fairly easy to enable for other archs.
-
 - CONFIG_SYS_BOOTM_LEN:
 		Normally compressed uImages are limited to an
 		uncompressed size of 8 MBytes. If this is not enough,