Add a basic version of the device_path protocol

Since the device_path protocol structure is really a header for several
types of structures, I'll have to think more about how to fully
implement it. Probably will have to put the dervied versions in
non-gerneated headers.

protos.yaml

@@ -2,6 +2,7 @@
   guid: "0x5b1b31a1,0x9562,0x11d2,{0x8e,0x3f,0x00,0xa0,0xc9,0x69,0x72,0x3b}"
   headers:
     - uefi/tables.h
+    - uefi/protos/device_path.h
   pre_data:
     - name: revision
       type: uint32_t
@@ -34,6 +35,16 @@
         - name: image_handle
           type: uefi::handle
 
+- name: device_path
+  guid: "0x09576e91,0x6d3f,0x11d2,{0x8e,0x39,0x00,0xa0,0xc9,0x69,0x72,0x3b}"
+  pre_data:
+    - name: type
+      type: uint8_t
+    - name: sub_type
+      type: uint8_t
+    - name: length
+      type: uint16_t
+
 - name: simple_text_output
   guid: "0x387477c2,0x69c7,0x11d2,{0x8e,0x39,0x00,0xa0,0xc9,0x69,0x72,0x3b}"
   headers: