mpc5200: Misc updates to a3m071 config header

This patch changes some features of the a3m071/a4m2k board support:

- Add bootcounter support
- Update MTD env default to correct values
- Add mtdparts to bootargs for mtd partitioning via kernel cmdline
- Added some default env variables for easy updating (kernel, dtb)
- Change README to the updated flash locations

Signed-off-by: Stefan Roese <sr@denx.de>