dt-bindings: i2c: cadence: Document `resets` property

The Cadence I2C controller has an external reset that needs to be
de-asserted before the I2C controller can be accessed.

Document the `resets` devicetree property that can be used to describe how
the reset signal is connected.

While the reset signal will always be present in hardware the devicetree
property is kept optional for backwards compatibility with existing systems
that do not specify the reset property and where the reset signal might not
be controlled by operating system.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Michal Simek <michal.simek@amd.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>