media: ov2680: Add support for more clk setups

On ACPI systems the following 2 scenarios are possible:

1. The xvclk is fully controlled by ACPI powermanagement, so there
   is no "xvclk" for the driver to get (since it is abstracted away).
   In this case there will be a "clock-frequency" device property
   to tell the driver the xvclk rate.

2. There is a xvclk modelled in the clk framework for the driver,
   but the clk-generator may not be set to the right frequency
   yet. In this case there will also be a "clock-frequency" device
   property and the driver is expected to set the rate of the xvclk
   through this frequency through the clk framework.

Handle both these scenarios by switching to devm_clk_get_optional()
and checking for a "clock-frequency" device property.

This is modelled after how the same issue was fixed for the ov8865 in
commit 73dcffeb ("media: i2c: Support 19.2MHz input clock in ov8865").

Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Reviewed-by: Tommaso Merciai <tomm.merciai@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>