arch/arm64: Support runtime frequency

Add ARM64 support for `CONFIG_TIMER_READS_ITS_FREQUENCY_AT_RUNTIME`.

Used for the `qemu_kvm_arm64` board. This should have been part
of #51932. Without this, the `qemu_kvm_arm64` board is painfully
slow due to the timer frequency being wrong.

Signed-off-by: Henri Xavier <datacomos@huawei.com>