KVM: arm64: selftests: Split arch_timer test code

KVM: arm64: selftests: Split arch_timer test code

Split the arch-neutral test code out of aarch64/arch_timer.c
and put them into a common arch_timer.c. This is a preparation
to share timer test codes in riscv.

Suggested-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Haibo Xu <haibo1.xu@intel.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>