perf mem/c2c: Document that SPE is used for mem and c2c on ARM

perf mem/c2c: Document that SPE is used for mem and c2c on ARM

Setup is non-trivial so also link to the full SPE docs.

Signed-off-by: James Clark <james.clark@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-perf-users@vger.kernel.or
Link: https://lore.kernel.org/r/20230124145929.557891-1-james.clark@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>