clocksource: hyper-v: Introduce TSC PFN getter

clocksource: hyper-v: Introduce TSC PFN getter

And rework the code to use it instead of the physical address, which isn't
required by itself.

This is a cleanup and precursor patch for upcoming support for TSC page
mapping into Microsoft Hypervisor root partition, where TSC PFN will be
defined by the hypervisor and not by the kernel.

Signed-off-by: Stanislav Kinsburskiy <stanislav.kinsburskiy@gmail.com>
CC: "K. Y. Srinivasan" <kys@microsoft.com>
CC: Haiyang Zhang <haiyangz@microsoft.com>
CC: Wei Liu <wei.liu@kernel.org>
CC: Dexuan Cui <decui@microsoft.com>
CC: Daniel Lezcano <daniel.lezcano@linaro.org>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: linux-hyperv@vger.kernel.org
CC: linux-kernel@vger.kernel.org
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Anirudh Rayabharam <anrayabh@linux.microsoft.com>
Link: https://lore.kernel.org/r/166749833420.218190.2102763345349472395.stgit@skinsburskii-cloud-desktop.internal.cloudapp.net
Signed-off-by: Wei Liu <wei.liu@kernel.org>