i2c: qup: Add device tree bindings information

The Qualcomm Universal Peripherial (QUP) wraps I2C mini-core and
provide input and output FIFO's for it. I2C controller can operate
as master with supported bus speeds of 100Kbps and 400Kbps.

Signed-off-by: Ivan T. Ivanov <iivanov@mm-sol.com>
[bjorn: reformulated part of binding description
        added version to compatible
        cleaned up example]
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Acked-by: Rob Herring <robh@kernel.org>
[wsa: removed the dummy child node which was a confusing example]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>