dt-bindings: apmu: Document r8a77470 support

Document APMU and SMP enable method for RZ/G1C (also known as
r8a77470) SoC.
Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

Documentation/devicetree/bindings/power/renesas,apmu.txt

@@ -9,6 +9,7 @@ Required properties:
 	      Examples with soctypes are:
 		- "renesas,r8a7743-apmu" (RZ/G1M)
 		- "renesas,r8a7745-apmu" (RZ/G1E)
+		- "renesas,r8a77470-apmu" (RZ/G1C)
 		- "renesas,r8a7790-apmu" (R-Car H2)
 		- "renesas,r8a7791-apmu" (R-Car M2-W)
 		- "renesas,r8a7792-apmu" (R-Car V2H)