dt-bindings: input: ti-tsc-adc: Add new compatible for AM654 SoCs

AM654 SoCs has ADC IP which is similar to AM335x, but without the
touchscreen part. Add new compatible to handle AM654 SoCs. Also, it
seems that existing compatible strings used in the kernel DTs were never
documented. So, document them now.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>
