media: i2c: ov2659: Use clk_{prepare_enable,disable_unprepare}() to set xvclk on/off

On some platform(imx6q), xvclk might not switch on in advance,
also for power save purpose, xvclk should not be always on.
so, add clk_prepare_enable(), clk_disable_unprepare() in driver
side to set xvclk on/off at proper stage.

Add following changes:
- add 'struct clk *clk;' in 'struct ov2659 {}'
- enable xvclk in ov2659_power_on()
- disable xvclk in ov2659_power_off()

Signed-off-by: Dillon Min <dillon.minfei@gmail.com>
Acked-by: Lad Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>