Input: tegra-kbc - revise device tree support

This is an incremental patch updating to the revised bindings for
matrix keyboards.

This includes an optional "linux,fn-keymap" binding that is not yet
implemented, that will be used to specify the Fn-key modifier layout
if needed.

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>