dt-bindings: i2c: i2c-gpio: Add support for named gpios

The current i2c-gpio DT bindings use a single unnamed "gpios" property
to refer to the SDA and SCL signal lines by index.  This is error-prone
for the casual DT writer and reviewer, as one has to look up the order
in the DT bindings.

Fix this by amending the DT bindings to use two separate named gpios
properties, and deprecate the old unnamed variant.

Take this opportunity to clearly deprecate the "i2c-gpio,sda-open-drain"
and "i2c-gpio,scl-open-drain" flags as well. The commit describes
in detail what these flags actually mean, and why they should not be
used in new device trees.

Cc: devicetree@vger.kernel.org
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
[Augmented to what I and Rob would like]
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>