powerpc/p1_p2_rdb_pc: Add a pin to reset the DDR chip for P1021RDB-PC

When P1021RDB-PC reboot system, the board will hung at uboot DDR
configuration. For P1021RDB-PC DDR reset pin is multiplex with
QE, so uboot will reserve this pin for QE and skip DDR reset.
Other platforms without QE will do this reset. This patch adds
a slight code to reset DDR chip by QE CE_PB8 pin for NAND and
NOR FLASH boot. For booting from SPI FALSH and SD card, it
seems possible to use the rom on chip to write to the GPIO
pins before configuring the DDR.

Signed-off-by: Xu Jiucheng <B37781@freescale.com>
Signed-off-by: Xie Xiaobo <X.Xie@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>