dt-bindings: gpio: davinci: Add keystone-k2g compatible

The patch adds keystone-k2g compatible, specific properties and
an example. The patch also adds the details of supported SoCs
for each compatible.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>