[ARM] 3/4 Rename common oprofile code

The common oprofile code assumes the name "PMU" (from Intel's
performance management unit).  This is misleading when we
start adding oprofile support for other machine types which
don't use the same terminology.  Call it op_arm_* instead of
pmu_*.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>