dt-bindings: display: ltk500hd1829: Remove the reg property

Commit 52120e8c ("dt-bindings: display: fix panel warnings") removed
the dsi unit name, but missed to remove the 'reg' property, which causes
the following 'make dt_binding_check' warning:

Documentation/devicetree/bindings/display/panel/leadtek,ltk500hd1829.example.dts:17.5-29.11: Warning (unit_address_vs_reg): /example-0/dsi: node has a reg or ranges property, but no unit name

Fix it by removing the unneeded 'reg' property.

Fixes: 52120e8c

 ("dt-bindings: display: fix panel warnings")
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200328183641.11226-1-festevam@gmail.com