arm64/perf: Add branch stack support in struct arm_pmu

This updates 'struct arm_pmu' for branch stack sampling support later. This
adds a new 'features' element in the structure to track supported features,
and another 'private' element to encapsulate implementation attributes on a
given 'struct arm_pmu'. These updates here will help in tracking any branch
stack sampling support, which is being added later. This also adds a helper
arm_pmu_branch_stack_supported().

This also enables perf branch stack sampling event on all 'struct arm pmu',
supporting the feature but after removing the current gate that blocks such
events unconditionally in armpmu_event_init(). Instead a quick probe can be
initiated via arm_pmu_branch_stack_supported() to ascertain the support.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>