dt-bindings: display: ti, tfp410.yaml: Redefine ti, deskew property

This changes how the 'ti,deskew' property is defined. It's now an
unsigned value from 0 to 7 instead of a signed value from -4 to 3.
Until the dtc carries the integer sign through to the yaml output it's
easier to define signed types as unsigned.

Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200617094633.19663-3-ricardo.canuelo@collabora.com