While accessing the ACPI/UEFI specifications still require jumping through some hoops, the ACPI 5.1 update is reported to fix major gaps in supporting ACPI on ARM. Hanjun Guo has already laid out patches for providing Linux ARM64 support compliant with the ACPI 5.1 specification. ACPI 5.1 has "major changes" to the MADT, FADT, GTDT, and _DSD for bettering up this non-x86 platform support.
Hanjun Guo sent out 19 patches this morning for introducing ACPI on 64-bit ARM. Developers interested in all of the ACPI AArch64 technical details can see the mailing list post. Testing of these patches from a 64-bit ARM "Juno" reference platform have shown that Linux can now boot successfully just using ACPI without any Device Tree dependence.
Also sent out were ACPICA updates for ACPI 5.1 as a temporary measure for testing the ARM64 support; a proper ACPI 5.1 update to ACPICA is being worked on by the upstream kernel developers. Hopefully with Linux 3.17 we'll be able to start seeing some updates for the ACPI 5.1 specification.