media: mt9m111: add regulator support

In the soc_camera removal, the board specific power callback was
dropped. This at least will remove the power optimization from ezx and
em-x270 pxa based boards.

As to recreate the same level of functionality, make the mt9m111 have a
regulator providing it its power, so that board designers can plug in a
gpio based or ldo regulator, mimicking their former soc_camera power
hook.

[sakari.ailus@linux.intel.com: fix a build warning]
Fixes: 5c10113cc668 ("media: mt9m111: make a standalone v4l2 subdevice")
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Tested-by: Akinobu Mita <akinobu.mita@gmail.com>
Tested-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>