Minor mips reorg and plans for the future

In trying to bring the MIPS tree more into line with idealized FreeBSD practices, I've moved the SoC directories up one level in the hierarchy. Future changes will make it easier to define different boards, as well as stylizing these sorts of things across different CPU architectures. I need this for the Plat'Home OpenMicroServer work I've started on, so I'll use that to drive these changes.

For the curious, I've moved mips/mips32/adm5120 -> mips/adm5120, likewise with the idt, malta, and sentry5 directories. While all of these were, strictly speaking, mips32 CPUs, having them in their own subdirectory doesn't make a lot of sense. There's never going to be so many of them that having an extra layer of indirection is going to help. Also, mips32 is ambiguous. Does it mean the MIPS ISA known as MIPS32 and MIPS32r2? Does it mean that the ports are running in 32-bit mode? It wasn't clear where a 32-bit kernel for a mips64 processor would live. Also, the idt parts aren't completely mips32 compliant in a couple of minor details. Is that non-compliance enough to move them elsewhere?

The next set of changes will try to tease apart the boot loader environment from the SoC that it is running on. While many SoCs have their own bootloader, often times one may also run uboot on the same hardware. In addition, things like uboot are used in many different platforms, so duplicating that code will grow tiresome.

Once the boot loader stuff is better abstracted, the next set of problems will be how to create a system that can run on multiple different boards of the same SoC. At first, this will be statically compiled, but eventually it will be dynamic.

Another key component of this work, whose overall goal is to make it easier to add board support to FreeBSD embedded platforms, will be an overhaul of the hints system. While it can be made to work, there are some practical problems it solves poorly that are desirable to have solved better. For example, it has no good way to tie attributes or parameters to hardware. Today, one can only tie them to a driver instance, which itself can be tied to hardware.

Finally, the last planned component in this work is a simplification of creation of busses with attributes and fixed or semi-fixed children. You can't have variants of hints loaded today that say "If I'm running on a AT91RM9200, use this set, but if I'm runnning on an AT91SAM9620 use this other set of hints" to enumerate the busses. This makes it hard to achieve the dynamic selection of boot code to run needed to allow kernels to boot on a variety of boards.

Most of this is low-level code janitor stuff. However, it is code janitor stuff that's needed and will be important to FreeBSD's success in the future.