Makefile: Drop include/asm directory as well as symlink

At present when using 'make mrproper' on an out-of-tree build, a warning
is shown about include/asm being a directory. With old versions of U-Boot
it is a file, but more recently it has become a directory.

Remove this directory first, since that covers both cases.
Signed-off-by: NSimon Glass <sjg@chromium.org>

Makefile

@@ -2100,7 +2100,7 @@ CLEAN_FILES += include/bmp_logo.h include/bmp_logo_data.h tools/version.h \
 
 # Directories & files removed with 'make mrproper'
 MRPROPER_DIRS  += include/config include/generated spl tpl \
-		  .tmp_objdiff doc/output
+		  .tmp_objdiff doc/output include/asm
 
 # Remove include/asm symlink created by U-Boot before v2014.01
 MRPROPER_FILES += .config .config.old include/autoconf.mk* include/config.h \