ARM: dts: am437x: cm-t43: add touchscreen support

AM437x has an internal touchscreen controller. Add support for it
on cm-t43.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Benoit Cousson <bcousson@baylibre.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Dmitry Lifshitz <lifshitz@compulab.co.il>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>