dt-bindings: i2c: iproc: make 'interrupts' optional

Update the binding document to make the 'interrupts' property optional.
For certain revisions of the I2C controller (e.g., iProc NIC I2C), I2C
interrupt is unwired to the interrupt controller. In such case, this
'interrupts' property should be left unspecified, and driver will fall
back to polling mode

Signed-off-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>