dt-bindings: i2c: riic: document r7s9210 support

Document support for the R7S9210 (RZ/A2) SoC. Also explicitly document
bindings for the R7S72100 (RZ/A1) SoC.
Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

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

 Device tree configuration for Renesas RIIC driver
 
 Required properties:
-- compatible      : "renesas,riic-<soctype>". "renesas,riic-rz" as fallback
+- compatible      :
+	"renesas,riic-r7s72100" if the device is a part of a R7S72100 SoC.
+	"renesas,riic-r7s9210" if the device is a part of a R7S9210 SoC.
+	"renesas,riic-rz" for a generic RZ/A compatible device.
 - reg             : address start and address range size of device
 - interrupts      : 8 interrupts (TEI, RI, TI, SPI, STI, NAKI, ALI, TMOI)
 - clock-frequency : frequency of bus clock in Hz