arm: mvebu: spl: Add option to reset the board on DDR training failure

Some boards may occacionally fail DDR training. Currently we hang() in
this case. Add an option that makes the board do an immediate reset in
such a case, so that a new training is tried as soon as possible,
instead of hanging and possibly waiting for watchdog to reset the board.

(If the DDR training fails while booting the image via UART, we will
 still hang - it doesn't make sense to reset in such a case, because
 after reset the board will try booting from another medium, and the
 UART booting utility does not expect that.)

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>