dt-bindings: input: Create macros for cros-ec keymap

In Chrome OS, the keyboard matrix can be split to two groups:

The keymap for the top row keys can be customized based on OEM
preference, while the keymap for the other keys is generic/fixed
across boards.

This patch creates marcos for the keymaps of these two groups, making
it easier to reuse the generic portion of keymap when we override the
keymap in the board-specific dts for custom top row design.

Signed-off-by: Philip Chen <philipchen@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20210115143555.v6.1.Iaa8a60cf2ed4b7ad5e2fbb4ad76a1c600ee36113@changeid


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