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/cmd/gpio.c b/cmd/gpio.c
index 5f4c7ff..8ce8ba9 100644
--- a/cmd/gpio.c
+++ b/cmd/gpio.c
@@ -10,6 +10,7 @@
 #include <command.h>
 #include <errno.h>
 #include <dm.h>
+#include <malloc.h>
 #include <asm/gpio.h>
 #include <linux/err.h>