[dt] Add DT_ROM() and DT_ID() macros

Add DT_ROM() and DT_ID() macros following the pattern for PCI_ROM()
and PCI_ID(), to allow for the possibility of including devicetree
network devices within the "all-drivers" build of iPXE.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/devtree.h

@@ -27,6 +27,12 @@ struct dt_device {
 	void *priv;
 };
 
+/* Define a devicetree device ID */
+#define DT_ID( _name, _desc ) _name
+
+/* Define a devicetree device ID with a corresponding build rule */
+#define DT_ROM( _name, _desc ) DT_ID ( _name, _desc )
+
 /** A devicetree driver */
 struct dt_driver {
 	/** Driver name */