Replace multiple nearly-identical copies of code to walk through an FDT
node's interrupts=<...> property creating resource list entries with a
single common implementation. This change makes ofw_bus_intr_to_rl() the
one true copy of that code and removes the copies of it from other places.

This also adds handling of the interrupts-extended property, which allows
specifying multiple interrupts for a node where each interrupt can have a
separate interrupt-parent. The bindings for this state that the property
cells contain an xref phandle to the interrupt parent followed by whatever
interrupt info that parent normally expects. This leads to having a
variable number of icells per interrupt in the property. For example you
could have <&intc1 1 &intc2 26 9 0 &intc3 9 4>.

Differential Revision: https://reviews.freebsd.org/D803