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/net/mdio-uclass.c b/net/mdio-uclass.c
index f75e4df..8e78721 100644
--- a/net/mdio-uclass.c
+++ b/net/mdio-uclass.c
@@ -6,9 +6,12 @@
 
 #include <common.h>
 #include <dm.h>
+#include <malloc.h>
 #include <miiphy.h>
 #include <dm/device-internal.h>
+#include <dm/device_compat.h>
 #include <dm/uclass-internal.h>
+#include <linux/compat.h>
 
 /* DT node properties for MAC-PHY interface */
 #define PHY_MODE_STR_CNT	2