docs/fb: Document current named modes

KMS supports a number of named modes already, but it's never been
documented anywhere, let's fix that.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://lore.kernel.org/r/20220728-rpi-analog-tv-properties-v9-1-24b168e5bcd5@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>