powerpc/85xx: Adds Support for P2020RDB-PC board

P2020RDB-PC Board shares the same design(PCB) as P102x RDB style platforms.
The difference between this platform and the already existing P2020RDB
is mainly with respect to DDR. The P2020RDB-PC has a DDR3 memory.
The P2020RDB-PC also has a CPLD device connected to local bus.

The main differences from the P102x RDB-PC is 64-bit DDR and SYSCLK of
100Mhz.

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Signed-off-by: Poonam Aggrwal <poonam.aggrwal@freescale.com>
Signed-off-by: Tang Yuantian <b29983@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>