[U-Boot] powerpc/85xx:debugger support patch

[Prabhakar Kushwaha]

This series of patches include 4 parts to overcome the limitation of 
e500 and e500v2 processor to support any debugger.

1. Add documentation for mpc85xx debugger support

2. powerpc/85xx:Fix MSR[DE] bit in MSR to support debugger

3. powerpc/85xx:Update NOR code base to support debugger

4. powerpc/85xx:Update NAND code base to support debugger

Signed-off-by: Prabhakar Kushwaha <prabhakar at freescale.com>