powerpc/freescale: Convert selected boards to generic board architecture

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 <[email protected]>
CC: Ying Zhang <[email protected]>
CC: Prabhakar Kushwaha <[email protected]>
CC: Haijun.Zhang <[email protected]>
CC: Scott Wood <[email protected]>
CC: Shaohui Xie <[email protected]>