arm: perf: factor out xscale pmu driver

Now that the core arm perf code maintains no global state and all
microarchitecture-specific PMU data can be fed in through the shared
probe function, it's possible to use it as a library and get rid of the
C file includes we have currently.

This patch factors out the xscale-specific portions out into the xscale
driver. For the moment this is always built if perf event support is
enabled, but the preprocessor guards will leave behind an empty file.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>