Marvell Dove SoC also has an UART bootmode to upload payload after
powerup. In contrast to Kirkwood and Armada 370/XP, Dove does not
respond to a special sequence sent on UART0 but requires to be
booted into UART bootmode by bootstraps. This is usually done by
pressing a push button that will set bootstraps accordingly.

This patch adds a new option, documentation and corresponding prompts
to support the above requirements. Also a left-over option ('-p')
is removed.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>