dm: core: Add a place to put extra device-tree reading functions

Some functions deal with structured data rather than simple data types.
It makes sense to have these in their own file. For now this

dm: core: Add a place to put extra device-tree reading functions

Some functions deal with structured data rather than simple data types.
It makes sense to have these in their own file. For now this just has a
function to read a flashmap entry. Move the data types also.

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

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