Attached device enumeration and boot firmware are somewhat orthogonal concerns; though certain ARM standards for various boot certifications ("ServerReady") do, in fact, require ACPI. But for mobile phones there's no need for it. You can just use a device-tree for the system and be done with it. (And yes, there are a lot of device tree files needed for all the devices, and while "self describing" would be nice it's ultimately a lot easier to just "have a lot of small files")