dm: core: Add macros to access the new linker lists

Add macros which work with instantiated devices and uclasses, as created
at build time by dtoc. Include variants that can be used in data
structures.

These are mostly used by dtoc but it is worth documenting them fully for
the occasional case where they might come up in user code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>