Add the device tree binding documentation for Apalis TK1.

Note that this is using dashes aka '-' in compatible strings as
previously suggested by Rob.
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>