ACPI: FPDT: expose FBPT and S3PT subtables via sysfs

ACPI: FPDT: expose FBPT and S3PT subtables via sysfs

Add sysfs files at /sys/firmware/acpi/fpdt/FBPT and
/sys/firmware/acpi/fpdt/S3PT that expose the raw contents of the
FPDT subtables.

Note that /sys/firmware/acpi/tables/FPDT only provides the top level
table, not the subtables. Adding access to the subtables enables a
usage model similar to /sys/firmware/dmi/tables/DMI, allowing
userspace tools to interpret newer record types (e.g. String Event
Records, Microcontroller Boot Performance Data Records, etc.) defined
in recent ACPI specifications [1] without requiring kernel changes.

Link: https://uefi.org/specs/ACPI/6.6/05_ACPI_Software_Programming_Model.html#performance-event-record-types
Signed-off-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
[ rjw: Changelog edits ]
Link: https://patch.msgid.link/20260324231456.701-2-nathaniel.l.desimone@intel.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>