doc/ramboot.mpc85xx: Documented the RAMBOOT for MPC85xx

doc/ramboot.mpc85xx: Documented the RAMBOOT for MPC85xx

There could be scenarios where the user would like to manually(via JTAG)
configure the DDR/L2SRAM and load the bootloader binary onto DDR/L2SRAM.
This document explains thse usecases and the detailed explanation of what needs
to be done to use it.

Most of the code from CONFIG_SYS_RAMBOOT will be used except for small changes
of CCSRBAR etc.

The changes are not very large, but it is good to document them so that user
can get it working at once.

Signed-off-by: Poonam Aggrwal <poonam.aggrwal@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>