arm: perf: factor out armv7 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 ARMv7-specific portions out into the ARMv7
driver. For the moment this is always built if perf event support is
enabled, but the preprocessor guards will leave behind an empty file.

Now that perf_event_cpu.c contains no microarchitecture-specific data,
the associated probing code is removed, completing its relegation to a
library file. The vestigal "arm-pmu" platform device ID is removed in
this patch, as it has been unused since platform files were updated to
specify a more specific PMU variant.

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