SUMMARY:
This partially cleans up the interaction between the sensor drivers and ov511.c,
to make implementation of standard kernel I2C easier.

PER-FILE CHANGES:
- Makefile:
  - Add support for ovsensor.c

- ov511.c:
  - Move logic for deciding whether streaming needs to be stopped during a
    sensor command into sensor_needs_stop()
  - Replace most sensor_get_* and sensor_set_* functions with generic
    sensor_[set|get]_control() functions.

- [All OmniVision sensor files]
  - Move ovsensor_write_regvals() to ovsensor.c
  - Split *_command() functions into separate set_control and get_control
    functions
  - Add new command function, which is roughly equivalent to ioctl(). This will
    eventually be the only entry point to the sensor drivers.

- ovsensor.c:
  - File added

- ovsensor.h:
  - Replace OVSENSOR_CMD_[S|G]* with OVSENSOR_CID*
  - Add ovsensor_control struct, which is functionally equivalent to
    struct v4l2_control