drivers: perf: arm_pmu: Add new sched_task() callback

This adds armpmu_sched_task(), as generic pmu's sched_task() override which
in turn can utilize a new arm_pmu.sched_task() callback when available from
the arm_pmu instance. This new callback will be used while enabling BRBE in
ARMV8 PMU.

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>