ACPI: Constify ACPI device IDs in documentation

ACPI device ID arrays normally don't need to be written to as they're
only ever read. The common usage -- embedding pointers to acpi_device_id
arrays in other data structures -- reference them as 'const', e.g. as in
struct acpi_driver / acpi_scan_handler / device_driver. The matchers are
taking const pointers, too. So it's only natural, to propose using const
arrays. Change the documentation accordingly.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>