media: dt-bindings: Add OF properties to ov7670

Describe newly introduced OF properties for ov7670 image sensor.
The driver supports two standard properties to configure synchronism
signals polarities and one custom property already supported as
platform data options to suppress pixel clock during horizontal
blankings.

Re-phrase child nodes description while at there.

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