board: freescale: p1_p2_rdb_pc: Avoid usage of CPLD's system reset register

CPLD's system reset register is buggy and requires workaround in U-Boot.
So use this kind of board reset only when there is no other reset option.

Introduce a new board_reset_last() callback which is last-stage
board-specific reset and implement CPLD's system reset in this new
board_reset_last() callback instead of board_reset() callback.

Fixes: 20fb58fc ("board: freescale: p1_p2_rdb_pc: Implement board_reset()")
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>