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/drivers/spi/spi-mem.c b/drivers/spi/spi-mem.c
index b8cf8dd..e900c99 100644
--- a/drivers/spi/spi-mem.c
+++ b/drivers/spi/spi-mem.c
@@ -12,6 +12,7 @@
 #include <linux/pm_runtime.h>
 #include "internals.h"
 #else
+#include <dm/device_compat.h>
 #include <spi.h>
 #include <spi-mem.h>
 #endif