ARM: implement mrc()/mcr() as DPM ops

ARM: implement mrc()/mcr() as DPM ops

Instead of having separate ARM11 and Cortex-A8 implementations of
this code, have one shared implementation which just builds on the
existing "run instruction via R0" support.

This enables followup patches to remove that now-unused code from
those two drivers.  (Patches to move the "mrc" and "mcr" code into
"struct arm" are due too ... MIPS and other cores do not support
those ARM-specific concepts.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>