Draft: hi846: add 816x612 full-sensor mode

an attempt to add the full-sensor mode with 4x binning. shows almost only noise. (TODO spend more time on this)

Fixes: https://source.puri.sm/Librem5/linux-next/-/issues/323

to recap, that's an example script for overwriting a few registers (with only 1 reg/value entry as an example, you can add a whole list, "redo" what the driver does by default and change whatever you want):

#!/bin/bash
path="/sys/kernel/debug/hi846/"

echo 1 > "$path/clear"

# these register values are overwriting the default settings when
# starting to stream
#     REG                                VALUE
echo "0x0902" > "$path/address" && echo "0x4319" > "$path/add_value"
# you can put a reg/value table here