devicetree: deprecate DT_LABEL variants

Deprecate DT_LABEL and DT_INST_LABEL as we have phased out
general 'label' usage and there isn't a need to keep a specific
set of macros for 'label'.  DT_PROP(node, label) works fine for
the small handful of cases that need it now.

Signed-off-by: Kumar Gala <galak@kernel.org>