powerpc/85xx: p2020rdb - move the NAND address.

It is not at 0xffa00000. According to current u-boot source the NAND
controller is always at 0xff800000 and it is either at CS0 or CS1
depending on NAND or NAND+NOR mode. In 36bit mode it is shifted to
0xfff800000 but it has always an eight there and never an A.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>