[acpi] Export a power button HID device

This lets us push the logic for handling a power button press outside of
the ACPI driver. This will make it simpler to perform more complex
actions, like triggering a proper shutdown through devmgr.

ZX-2483 #comment Export the power button from ACPI

Test: Booted on qemu, NUC, and pixelbook, used "hid read" to see the
power button events.

Change-Id: I61678fb773fa4dcf190fc5d32ea51d1feb177852