media: dt: bindings: Add strobe property for CCP2

Document the CSI1/CCP2 property strobe. It signifies the clock or
strobe mode.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Sebastian Reichel <sre@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>