acpi(4): New iattr `apeibus' for attaching an APEI driver.

APEI is the ACPI Platform Error Interface, a standard (if very
complicated) interface for reporting hardware errors to the OS.

Firmware support for APEI is presented through the ACPI tables BERT
(Boot Error Record Table), ERST (Error Record Serialization Table),
EINJ (Error Injection Table), and HEST (Hardware Error Source Table),
rather than through nodes in the ACPI device tree, so it can't just
attach through the existing acpinodebus iattr and instead requires a
special pseudo-bus like acpiwdrt(4).

No driver yet -- this is just the hook to attach one in a module.

The new member sc_apei of struct acpi_softc is placed at the end of
the structure so that this change can be safely pulled up to release
branches without risk to ABI compatibility in existing modules such
as acpiverbose.kmod which may rely on the layout (but not size) of
struct acpi_softc.

PR kern/58046

(riastradh)