dt-bindings: display: bridge: lt8912b: Drop reset gpio requirement

Drop the gpio reset from the list of required properties. The bridge works
fine with a reset not managed by Linux. In the driver itself the gpio is
already flagged as optional.

Signed-off-by: Dominik Haller <d.haller@phytec.de>
Link: https://patch.msgid.link/20260130205820.83189-6-d.haller@phytec.de
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>