XEN / ACPI: Make XEN ACPI depend on X86

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 <[email protected]>
CC: Konrad Rzeszutek Wilk <[email protected]>
CC: Boris Ostrovsky <[email protected]>
CC: David Vrabel <[email protected]>
Acked-by: Stefano Stabellini <[email protected]>
Signed-off-by: Hanjun Guo <[email protected]>
Signed-off-by: Will Deacon <[email protected]>