ARM: perf: add support for the Cortex-A12 PMU

Cortex-A12 implements Performance Monitors compliant with the PMUv2
architecture.

This patch adds support for the Cortex-A12 PMU to the ARM perf backend.

Signed-off-by: Albin Tonnerre <albin.tonnerre@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>