x86/acpi: Add acpi_get_cpu_uid() for unified ACPI CPU UID retrieval

x86/acpi: Add acpi_get_cpu_uid() for unified ACPI CPU UID retrieval

As a step towards unifying the interface for retrieving ACPI CPU UID
across architectures, introduce a new function acpi_get_cpu_uid() for
x86. While at it, add input validation to make the code more robust.

Update Xen-related code to use acpi_get_cpu_uid() instead of the legacy
cpu_acpi_id() function, and remove the now-unused cpu_acpi_id() to clean
up redundant code.

Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://patch.msgid.link/20260401081640.26875-5-fengchengwen@huawei.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>