Split out the FDT attachment from the gic driver

[andrew]

Summary

Prepare for adding an ACPI attachment to the GIC driver by splitting out
the FDT parts of it to a new attachment. There are still a few FDT bits
left that are needed for both INTRNG and the old interrupt code, however
these are difficult to remove in a clean way.

Test Plan

Booted to multi user on QEMU with both INTRNG and the old interrupt code