v3/ec/docs: update docs with new functionality

Update the main README to remove notes about some of the chips on
the I2C_1 bus, and to clarify that `stm32reset` will be going away
on Proto 1.
Update the videomux documentation to show the DisplayPort to IT68051
functionality working.

BUG=b:162915510
TEST=Build the firmware as directed in README and flash to the board.
Do the steps in the "DisplayPort to IT68051" section of videomux to
see it working.

Change-Id: I8d4ac7cdfb127177fb223af12e7f35cd1b42983a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/chameleon/+/2743530
Tested-by: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: Alexandru M Stan <amstan@chromium.org>
Commit-Queue: Paul Fagerburg <pfagerburg@chromium.org>
5 files changed