dt-bindings: input: cros-ec-keyb: Add a new property describing top row

Add a new property `function-row-physmap` to the
device tree for the custom keyboard top row design.

The property describes the rows/columns of the top row keys
from left to right.

Signed-off-by: Philip Chen <philipchen@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210115122412.v7.1.I025fb861cd5fa0ef5286b7dce514728e9df7ae74@changeid


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>