Platform Runtime Mechanism (PRM) is a firmware interface that exposes
a set of binary executables that can either be called from the AML
interpreter or device drivers by bypassing the AML interpreter.
This change implements the AML interpreter path.

According to the specification [1], PRM services are listed in an
ACPI table called the PRMT. This patch parses module and handler
information listed in the PRMT and registers the PlatformRtMechanism
OpRegion handler before ACPI tables are loaded.

Each service is defined by a 16-byte GUID and called from writing a
26-byte ASL buffer containing the identifier to a FieldUnit object
defined inside a PlatformRtMechanism OperationRegion.

    OperationRegion (PRMR, PlatformRtMechanism, 0, 26)
    Field (PRMR, BufferAcc, NoLock, Preserve)
    {
        PRMF, 208 // Write to this field to invoke the OperationRegion Handler
    }

The 26-byte ASL buffer is defined as the following:

Byte Offset   Byte Length    Description
=============================================================
     0             1         PRM OperationRegion handler status
     1             8         PRM service status
     9             1         PRM command
    10            16         PRM handler GUID

The ASL caller fills out a 26-byte buffer containing the PRM command
and the PRM handler GUID like so:

    /* Local0 is the PRM data buffer */
    Local0 = buffer (26){}

    /* Create byte fields over the buffer */
    CreateByteField (Local0, 0x9, CMD)
    CreateField (Local0, 0x50, 0x80, GUID)

    /* Fill in the command and data fields of the data buffer */
    CMD = 0 // run command
    GUID = ToUUID("xxxx-xx-xxx-xxxx")

    /*
     * Invoke PRM service with an ID that matches GUID and save the
     * result.
     */
    Local0 = (\_SB.PRMT.PRMF = Local0)

Byte offset 0 - 8 are written by the handler as a status passed back to AML
and used by ASL like so:

    /* Create byte fields over the buffer */
    CreateByteField (Local0, 0x0, PSTA)
    CreateQWordField (Local0, 0x1, USTA)

In this ASL code, PSTA contains a status from the OperationRegion and
USTA contains a status from the PRM service.

The 26-byte buffer is recieved by acpi_platformrt_space_handler. This
handler will look at the command value and the handler guid and take
the approperiate actions.

Command value    Action
=====================================================================
    0            Run the PRM service indicated by the PRM handler
                 GUID (bytes 10-26)

    1            Prevent PRM runtime updates from happening to the
                 service's parent module

    2            Allow PRM updates from happening to the service's parent module

This patch enables command value 0.

Link: https://uefi.org/sites/default/files/resources/Platform%20Runtime%20Mechanism%20-%20with%20legal%20notice.pdf # [1]
Signed-off-by: Erik Kaneda <erik.kaneda@intel.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>