Add ARM64-specific code to initialize the Hyper-V
hypervisor when booting as a guest VM.

This code is built only when CONFIG_HYPERV is enabled.
Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/1628092359-61351-4-git-send-email-mikelley@microsoft.comSigned-off-by: Wei Liu <wei.liu@kernel.org>