doc: board: amlogic: add documentation on boot flow

This is a preliminary documentation introducing different
boot sequences, and notably the recovery mode.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Link: https://lore.kernel.org/r/20220706093649.3261229-1-narmstrong@baylibre.com