dm: core: Create a new header file for 'compat' features

At present dm/device.h includes the linux-compatible features. This
requires including linux/compat.h which in turn includes a lot of headers.
One of these is malloc.h which we thus end up including in every file in
U-Boot. Apart from the inefficiency of this, it is problematic for sandbox
which needs to use the system malloc() in some files.

Move the compatibility features into a separate header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/libavb/avb_descriptor.c b/lib/libavb/avb_descriptor.c
index 9f03b97..86b8d1b 100644
--- a/lib/libavb/avb_descriptor.c
+++ b/lib/libavb/avb_descriptor.c
@@ -6,6 +6,7 @@
 #include "avb_descriptor.h"
 #include "avb_util.h"
 #include "avb_vbmeta_image.h"
+#include <malloc.h>
 
 bool avb_descriptor_validate_and_byteswap(const AvbDescriptor* src,
                                           AvbDescriptor* dest) {