ARM: oprofile: Move non-ARM code into separate init/exit

In preparation for moving the majority of this oprofile code into an
architecture-neutral place separate the architecture-independent code
into oprofile_perf_init() and oprofile_perf_exit().

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Tested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Robert Richter <robert.richter@amd.com>