Add two new interfaces to ofw_bus:

• ofw_bus_map_intr() Maps an (iparent, IRQ) tuple to a system-global interrupt number in some platform dependent way. This is meant to be implemented as a replacement for [FDT_]MAP_IRQ() that is an MI interface that knows about the bus hierarchy.
• ofw_bus_config_intr() Configures an interrupt (previously mapped) based on firmware sense flags. This replaces manual interpretation of the sense field in bus drivers and will, in a follow-up, allow that interpretation to be redirected to the PIC drivers where it belongs. This will eventually replace the tables in /sys/dev/fdt/fdt_ARCH.c

The PowerPC/AIM code has been converted to use these globally, with an
implementation in terms of MAP_IRQ() and powerpc_config_intr(), assuming
OpenPIC, at the bus root in nexus(4). The ofw_bus_config_intr() will shortly
be integrated into pic_if.m and bounced through nexus into the PIC tree.

FDT integration will happen significantly later due to larger testing
requirements. This patch in general also lays the groundwork for the removal
of /sys/dev/fdt/fdt_ARCH.c and machine/fdt.h.