arm64: enable HAVE_IRQ_TIME_ACCOUNTING

arm64 relies on the arm_arch_timer for sched_clock, so we can select
HAVE_IRQ_TIME_ACCOUNTING and have the core sched-clock code enable the
feature at runtime based on the rate.

Reported-by: Mario Smarduch <m.smarduch@samsung.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>