arm: perf: move event map macros to pmu.h

A few PMU-related macros are now looking a little lonely in
asm/perf_event.h now that all other PMU-specific structs, function
prototypes and macros live in pmu.h.

So as to make their placement consistent and to make it easier to build
atop of the current PMU functionality, let's reunite the entire family in
pmu.h

Acked-by: Will Deacon <will.deacon@arm.com>
Tested-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>