dt-bindings: i2c: Convert UniPhier FI2C controller to json-schema

Convert the UniPhier FIFO-builtin I2C (a.k.a. FIFO) controller binding
to DT schema format.

There are two types of I2C controllers used on the UniPhier platform.
This is the new one with FIFO support, which is used on the Pro4 SoC
or newer.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>