Add a new header that defines the AMD ACPI interface used
for laptops, PowerXpress, and chipset specific functionality
and update the current code to use it.

Todo:
- properly verify the ACPI interfaces
- hook up and handle ACPI notifications
- make PX code more robust
- implement PCIe Gen and width switching using ACPI

v2: fix typo in header
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>