[POWERPC] mpc82xx: Remove a bunch of cruft that duplicates generic functionality.

m82xx_calibrate_decr(), mpc82xx_ads_show_cpuinfo(), and mpc82xx_halt() do
anything useful beyond what the generic code does.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>