dt-bindings: i2c: Convert apm,xgene-slimpro-i2c to DT schema

Convert APM X-Gene slimpro-i2c binding to DT schema format. It's a
straight-forward conversion.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>