loader: Add pnp functions for autoloading modules based on linker.hints

This adds some new commands to loader :

• pnpmatch This takes a pnpinfo string as argument and tries to find a kernel module associated with it. -v and -d option are available and are the same as in devmatch (v is verbose, d dumps the hints).
• pnpload This takes a pnpinfo string as argument and tries to load a kernel module associated with it.
• pnpautoload This will attempt to load every kernel module for each buses. Each buses are probed, the probe function will generate pnpinfo string and load kernel module associated with it if it exists.

Only simplebus for FDT system is implemented for now.
Since we need the dtb and overlays to be applied before searching the tree
fdt_devmatch_next will load and apply the dtb + overlays.

All the pnp parsing code comes from devmatch and is the same at 99%.

Reviewed by: imp, kevans
Differential Revision: https://reviews.freebsd.org/D19498