Add PIXIS FPGA support for MPC8641HPCN board.

Move the 8641HPCN's PIXIS code to the new directory
board/freescale/common/ as it will be shared by
future boards not in the same processor family.

Write a "pixis_reset" command that utilizes the FPGA
reset sequencer to support alternate soft-reset options
such as using the "alternate" flash bank, enabling
the watch dog, or choosing different CPU frequencies.

Add documentation for the pixis_reset to README.mpc8641hpcn.

Signed-off-by: Haiying Wang <haiying.wang@freescale.com>
Signed-off-by: Jon Loeliger <jdl@freescale.com>