powerpc/mpc8xxxx: FSL DDR debugger auto run of stored commands

This patch adds the ability for the FSL DDR interactive debugger to
automatically run the sequence of commands stored in the ddr_interactive
environment variable.  Commands are separated using ';'.

ddr_interactive=compute; edit c0 d0 dimmparms caslat_X 0x3FC0; go

Signed-off-by: James Yang <James.Yang@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>