media: dt-bindings: media: adv7604: Fix slave map documentation

The reg-names property in the documentation is missing an '='. Add it.

Fixes: 9feb7868 ("media: dt-bindings: media: adv7604: Extend
bindings to allow specifying slave map addresses")

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>