dt-bindings: i2c: dw: Add Baikal-T1 SoC I2C controller

Add the "baikal,bt1-sys-i2c" compatible string to the DW I2C binding. Even
though the corresponding node is supposed to be a child of the Baikal-T1
System Controller, its reg property is left required for compatibility.
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

Documentation/devicetree/bindings/i2c/snps,designware-i2c.yaml

@@ -31,6 +31,8 @@ properties:
         items:
           - const: mscc,ocelot-i2c
           - const: snps,designware-i2c
+      - description: Baikal-T1 SoC System I2C controller
+        const: baikal,bt1-sys-i2c
 
   reg:
     minItems: 1