dt-bindings: display: sitronix, st7735r: Make reset-gpios optional

There are other ways than using a gpio to reset the controller so make
this property optional.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211124150757.17929-3-noralf@tronnes.org