i2c: sh_mobile: document support for r8a77990 (R-Car E3)

Document support for the IIC code for the r8a77990 (R-Car E3).

It is not considered compatible with existing fallback bindings
due to the documented absence of automatic transmission registers.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

Documentation/devicetree/bindings/i2c/i2c-sh_mobile.txt

@@ -16,6 +16,7 @@ Required properties:
 			- "renesas,iic-r8a7795" (R-Car H3)
 			- "renesas,iic-r8a7796" (R-Car M3-W)
 			- "renesas,iic-r8a77965" (R-Car M3-N)
+			- "renesas,iic-r8a77990" (R-Car E3)
 			- "renesas,iic-sh73a0" (SH-Mobile AG5)
 			- "renesas,rcar-gen2-iic" (generic R-Car Gen2 or RZ/G1
 							compatible device)
@@ -28,7 +29,13 @@ Required properties:
 			the platform first followed by the generic R-Car
 			version.
 
-			renesas,rmobile-iic must always follow.
+			When compatible with "renesas,rmobile-iic" it should
+			be the last compatibility string listed.
+
+			The r8a77990 (R-Car E3) controller is not
+			considered compatible with "renesas,rcar-gen3-iic"
+			or "renesas,rmobile-iic" due to the absence of
+			automatic transmission registers.
 
 - reg             : address start and address range size of device
 - interrupts      : interrupt of device