bus: efi: add basic ACPI bus infrastructure ...

It makes sense to support some ACPI tables like the WDAT (Watchdog
Action Table) in barebox. To facilitate this add a ACPI bus and
the necessary bits to integrate ACPI tables into the barebox
device/driver model as devices. These devices are identified by the
four byte signature, which drivers can then match against and the
system description table (SDT) of the device is then available as a
device memory resource.

Even without drivers, with this patch, devinfo and md can now be used
to view the ACPI system description tables, which can be useful
during UEFI payload development.

Support for the ACPI Machine Language and ACPI 5.1 _DSD (Device
Specific Data) is not implemented.

Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>