media: ov08x40: Get clock on ACPI platforms too

ACPI platforms might also have a clk provider which needs to be controlled,
always try to get a clk using clk_get_optional() and when that fails fall
back to getting the xvclk frequency from the "clock-frequency" property.

Tested-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>