fdtdec: encapsulate dtb_dt_embedded() within

fdtdec: encapsulate dtb_dt_embedded() within

  Patch keeps the access to dtb_dt_embedded() within fdtdec API,
by means of new API function introduction. This new function is a
common place for updating appropriate global_data fields for
OF_EMBED case.

  Also, the consequence of the patch is movement of '___dtb_dt_*begin'
symbols' declaration from header file, because nobody used symbols
outside the lib/fdtdec.c.

Signed-off-by: Evgeny Bachinin <EABachinin@salutedevices.com>
Suggested-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>