drivers/perf: arm_pmu_platform: do not warn about affinity on uniprocessor

If there is exactly one CPU present, there is no ambiguity: do not warn
that PMU setup would need to guess IRQ affinity.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Alexander Monakov <amonakov@ispras.ru>
Signed-off-by: Will Deacon <will.deacon@arm.com>