media: v4l2: i2c: ov7670: Implement OF mbus configuration

ov7670 driver supports two optional properties supplied through platform
data, but currently does not support any standard video interface
property.

Add support through OF parsing for 2 generic properties (vsync and hsync
polarities) and for one custom property already supported through
platform data to suppress pixel clock output during horizontal
blanking.

While at there, check return value of register writes in set_fmt
function and rationalize spacings.

Signal polarities and pixel clock blanking verified through scope and
image capture.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>