clocksource: sh_tmu: Document r8a7778 binding

The r8a7778 is very similar to the r8a7779, and already handled by
the current driver in the non-DT case.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

Documentation/devicetree/bindings/timer/renesas,tmu.txt

@@ -10,6 +10,7 @@ Required Properties:
 
   - compatible: must contain one or more of the following:
     - "renesas,tmu-r8a7740" for the r8a7740 TMU
+    - "renesas,tmu-r8a7778" for the r8a7778 TMU
     - "renesas,tmu-r8a7779" for the r8a7779 TMU
     - "renesas,tmu" for any TMU.
       This is a fallback for the above renesas,tmu-* entries