board: freescale: p1_p2_rdb_pc: Turn off watchdog before reset

P1/P2 RDB boards have external max6370 watchdog connected to CPLD and this
watchdog is not deactivated on board reset. So if it is active during board
reset, it can trigger another reset when CPU is booting U-Boot. To prevent
possible infinite reset loop caused by external watchdog, turn it off
before reset.

Do it via a new board_reset_prepare() callback which is called from
do_reset() function before any reset sequence.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>