powerpc/mpc85xx:Add debugger support for e500v2 SoC

Freescale's e500v1 and e500v2 cores (used in mpc85xx chips) have some
restrictions on external debugging (JTAG).

So define CONFIG_SYS_PPC_E500_DEBUG_TLB to enable a temporary TLB entry to be
used during boot to work around the limitations.

Please refer doc/README.mpc85xx for more information

Signed-off-by: Radu Lazarescu <radu.lazarescu@freescale.com>
Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>