media: mt9m111: document missing required clocks property

The mt9m111 driver requires clocks property for the master clock to the
sensor, but there is no description for that.  This adds it.

Cc: Rob Herring <robh@kernel.org>
Cc: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>