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 <[e-mail]>
Signed-off-by: Will Deacon <[e-mail]>

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

···
     |       alpha: |  ..  |
     |         arc: | TODO |
     |         arm: |  ok  |
-    |       arm64: |  ..  |
+    |       arm64: |  ok  |
     |       avr32: | TODO |
     |    blackfin: | TODO |
     |         c6x: | TODO |

arch/arm64/Kconfig

···
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_GENERIC_DMA_COHERENT
 	select HAVE_HW_BREAKPOINT if PERF_EVENTS
+	select HAVE_IRQ_TIME_ACCOUNTING
 	select HAVE_MEMBLOCK
 	select HAVE_PATA_PLATFORM
 	select HAVE_PERF_EVENTS