drivers: iommu: hyperv: Make HYPERV_IOMMU only available on x86

Currently hyperv-iommu is implemented in a x86 specific way, for
example, apic is used. So make the HYPERV_IOMMU Kconfig depend on X86
as a preparation for enabling HyperV on architecture other than x86.

Cc: Lan Tianyu <Tianyu.Lan@microsoft.com>
Cc: Michael Kelley <mikelley@microsoft.com>
Cc: linux-hyperv@vger.kernel.org
Signed-off-by: Boqun Feng (Microsoft) <boqun.feng@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>