dev: Implement armv8pmu_branch_save()

This modifies current armv8pmu_sched_task(), to implement a branch records
save mechanism via armv8pmu_branch_save() when a task scheds out of a cpu.
BRBE is paused and disabled for all exception levels before branch records
get captured, which then get concatenated with all existing stored records
present in the task context maintaining the contiguity. Although the final
length of the concatenated buffer does not exceed implemented BRBE length.

Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>