dm: core: Support accessing core tags

dm: core: Support accessing core tags

At present tag numbers are only allocated for non-core data, meaning that
the 'core' data, like priv and plat, are accessed through dedicated
functions.

For debugging and consistency it is convenient to use tags for this 'core'
data too. Add support for this, with new tag numbers and functions to
access the pointer and size for each.

Update one of the test drivers so that the uclass-private data can be
tested here.

There is some code duplication with functions like device_alloc_priv() but
this is not addressed for now. At some point, some rationalisation may
help to reduce code size, but more thought it needed on that.

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