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>

Documentation/features/time/irq-time-acct/arch-support.txt

@@ -9,7 +9,7 @@
     |       alpha: |  ..  |
     |         arc: | TODO |
     |         arm: |  ok  |
-    |       arm64: |  ..  |
+    |       arm64: |  ok  |
     |       avr32: | TODO |
     |    blackfin: | TODO |
     |         c6x: | TODO |