perf tools arm64: Add support for get_cpuid_str function.

perf tools arm64: Add support for get_cpuid_str function.

The get_cpuid_str function returns the MIDR string of the first online
cpu from the range of cpus associated with the PMU CORE device.

Signed-off-by: Ganapatrao Kulkarni <ganapatrao.kulkarni@cavium.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ganapatrao Kulkarni <gklkml16@gmail.com>
Cc: Jayachandran C <jnair@caviumnetworks.com>
Cc: Jonathan Cameron <jonathan.cameron@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Robert Richter <robert.richter@cavium.com>
Cc: Shaokun Zhang <zhangshaokun@hisilicon.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/20171016183222.25750-3-ganapatrao.kulkarni@cavium.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>