ACPI / sysfs: Extend ACPI sysfs to provide access to boot error region

The ACPI sysfs interface provides a way to read each ACPI table from
userspace via entries in /sys/firmware/acpi/tables/

The BERT table simply provides the size and address of the error
record in BIOS reserved memory and users may want access to this
record.

In an earlier age we might have used /dev/mem to retrieve this error
record, but many systems disable /dev/mem for security reasons.

Extend this driver to provide read-only access to the data via a
file in a new directory /sys/firmware/acpi/tables/data/BERT

Acked-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>

v4: fix typo reported by Punit
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>