dt: snps,designware-i2c: Add clock bindings documentation

The driver requires an undocumented clock property, so detail it.
Add documentation for a separate, optional, interface clock.

Signed-off-by: Phil Edworthy <[e-mail]>
Signed-off-by: Gareth Williams <[e-mail]>
Acked-by: Wolfram Sang <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Reviewed-by: Jarkko Nikula <[e-mail]>
Signed-off-by: Wolfram Sang <[e-mail]>

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

···
                 or "mscc,ocelot-i2c" with "snps,designware-i2c" for fallback
  - reg : Offset and length of the register set for the device
  - interrupts : <IRQ> where IRQ is the interrupt number.
+ - clocks : phandles for the clocks, see the description of clock-names below.
+   The phandle for the "ic_clk" clock is required. The phandle for the "pclk"
+   clock is optional. If a single clock is specified but no clock-name, it is
+   the "ic_clk" clock. If both clocks are listed, the "ic_clk" must be first.
 
 Recommended properties :
 
  - clock-frequency : desired I2C bus clock frequency in Hz.
 
 Optional properties :
+
+ - clock-names : Contains the names of the clocks:
+    "ic_clk", for the core clock used to generate the external I2C clock.
+    "pclk", the interface clock, required for register access.
+
  - reg : for "mscc,ocelot-i2c", a second register set to configure the SDA hold
    time, named ICPU_CFG:TWI_DELAY in the datasheet.