media: dt-bindings: adv7604: Fix documentation for hpd-gpios

As of [1] the hpd-gpios property is optional, document it as such.

1. commit 269bd132 ("[media] media: adv7604: improve usage of gpiod API")

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>