powerpc/freescale: Convert selected boards to generic board architecture

This patch converts the following boards to use generic board: MPC8536DS,
MPC8572DS, MPC8641HPCN, p1_p2_rdb_pc, corenet_ds, t4qds, B4860QDS. It has
been tested on NOR boot on MPC8536DS, MPC8572DS, P1021RDB, P4080DS,
P5020DS, P5040DS, P3041DS, T4240QDS, B4860QDS.

Signed-off-by: York Sun <yorksun@freescale.com>
CC: Ying Zhang <b40530@freescale.com>
CC: Prabhakar Kushwaha <prabhakar@freescale.com>
CC: Haijun.Zhang <Haijun.Zhang@freescale.com>
CC: Scott Wood <scottwood@freescale.com>
CC: Shaohui Xie <Shaohui.Xie@freescale.com>