arm64: timer: Add support for phys timer testing

Rearrange the code to be able to reuse as much as posible and add
support for testing the physical timer as well.

Also change the default unittests configuration to run a separate vtimer
and ptimer test so that older kernels without ptimer support just show a
failure.

Signed-off-by: Christoffer Dall <cdall@linaro.org>
Message-Id: <20170713192009.10069-4-cdall@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>