drivers: video: add emulated Imager driver and RX driver

Add a new implementation of a test pattern generator, with the same
architecture as real drivers: split receiver core and
I2C-controlled sub-device, with changes of video format in
"zephyr,emul-imager" leads to different data produced by
"zephyr,emul-rx".

Signed-off-by: Josuah Demangeon <me@josuah.net>