[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. doc:Add documentation for e500 external debugger support

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

3. powerpc/85xx:Make debug exception vector accessible

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

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