media: ov5693: rename clk into xvclk

Rename clk pdata pointer into xvclk (system clock input).
Same for clk_rate into xvclk_rate. This is more explicit

Signed-off-by: Tommaso Merciai <tommaso.merciai@amarulasolutions.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>