dt-bindings: i2c: extend existing opencore bindings

Reformatted compatibility strings to one valid combination on
each line.
Add FU540-C000 specific device tree bindings to already available
i2-ocores file. This device is available on
HiFive Unleashed Rev A00 board. Move interrupt under optional
property list as this can be optional.

The FU540-C000 SoC from sifive, has an Opencore's I2C block
reimplementation.

The DT compatibility string for this IP is present in HDL and available at.
https://github.com/sifive/sifive-blocks/blob/master/src/main/scala/devices/i2c/I2C.scala#L73



Signed-off-by: Sagar Shrikant Kadam <sagar.kadam@sifive.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>