ACPI / LPSS: constify device descriptors

The device descriptors are never written to -- even pointed to as
'const' from struct lpss_private_data. Make them r/o for real.

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