media: imx: imx7_mipi_csis: Acquire reset control without naming it

The device has a single reset line, there's thus no need to name it
explicitly when calling devm_reset_control_get_exclusive(). Drop the
name in preparation for the removal of the reset-names property in the
DT binding.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>