dt-bindings: i2c: mv64xxx: Document DMA properties

Allwinner's I2C offload engine includes bidirectional DMA support. Add
the properties for describing this in the devicetree. "dmas" is optional
because not all instances of the controller have their DRQs hooked up.
For example, R_I2C0 and R_I2C1 on V536 have no DRQ number assigned.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>