ARM: prima2: convert to common sched_clock() implementation

Prima2 has its own sched_clock() implementation, which gets in the
way of a single zImage. Moving to the common sched_clock framework
makes the code slightly cleaner (the mapping hack in sched_clock()
goes away...).

Acked-by: Barry Song <baohua.song@csr.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>