media: mt9m111: allow to setup pixclk polarity

The chip can be configured to output data transitions on the
rising or falling edge of PIXCLK (Datasheet R58:1[9]), default is on the
falling edge.

Parsing the fw-node is made in a subfunction to bundle all (future)
dt-parsing / fw-parsing stuff.

[m.grzeschik@pengutronix.de: Fix inverting clock. INV_PIX_CLOCK bit is set
 per default. Set bit to 0 (enable mask bit without value) to enable
 falling edge sampling.]
[m.felsch@pengutronix.de: use fwnode helpers]
[m.felsch@pengutronix.de: mv fw parsing into own function]
[m.felsch@pengutronix.de: adapt commit msg]
[sakari.ailus@linux.intel.com: V4L2 API usage changes to compile
 on media tree master]

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>