ARM: dts: Create a cros-ec-keyboard fragment

All ChromeOS ARM devices that have the standard "CrOS EC" have the
same keyboard mapping.  It's silly to include this same definition
everywhere.  Let's create a "dtsi" fragment that we can include from
many different boards.

This fragment is based on what's currently in tegra124-venice2.dts

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>