XEN / ACPI: Make XEN ACPI depend on X86

When ACPI is enabled on ARM64, XEN ACPI will also compiled
into the kernel, but XEN ACPI is x86 dependent, so introduce
CONFIG_XEN_ACPI to make it depend on x86 before XEN ACPI is
functional on ARM64.

CC: Julien Grall <julien.grall@linaro.org>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: Boris Ostrovsky <boris.ostrovsky@oracle.com>
CC: David Vrabel <david.vrabel@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>