media: staging: dt-bindings: phy-rockchip-dphy-rx0: remove non-used reg property

reg property is not used in Rockchip MIPI DPHY RX0 bindings, thus remove
it.

Suggested-by: Johan Jonker <jbx6244@gmail.com>
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>