dev: Add two BRBE regset helper functions

The primary absttaction level for fetching branch records from BRBE HW has
been changed as 'struct brbe_regset', which contains storage for all three
BRBE registers i.e BRBSRC, BRBTGT, BRBINF. Whether branch record processing
happens in the task sched out path or in the PMU IRQ handling path, these
registers need to extracted from the HW, live and stored records need to be
concatenated together. This adds required helper functions in this regard.

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