ACPI / proc: make ACPI_PROCFS_POWER X86 only

The ACPI procfs power interface is initialized by compilation units
that are only selectable on X86 platforms. Since its usage is
deprecated and it cannot even be used on platforms other than X86
it should be compiled in only on X86 platforms.

This patch makes CONFIG_ACPI_PROCFS_POWER dependent on X86, so
that other architectures are prevented from compiling it in for
no purpose.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>