docs: driver-api: firmware: add driver firmware guidelines. (v3)

docs: driver-api: firmware: add driver firmware guidelines. (v3)

A recent snafu where Intel ignored upstream feedback on a firmware
change, led to a late rc6 fix being required. In order to avoid this
in the future we should document some expectations around
linux-firmware.

I was originally going to write this for drm, but it seems quite generic
advice.

v2: rewritten with suggestions from Thorsten Leemhuis
v3: rewritten with suggestions from Mauro

Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://lore.kernel.org/r/20220721044352.3110507-1-airlied@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>