Currently running dtbs_check compiles and runs the DT checker on all
enabled devicetrees against all dt-bindings. This can take a long time,
and is an unnecessary burden when just validating a new devicetree or
changes in an existing one, with the dt-bindings unchanged.
Similarly to DT_SCHEMA_FILES for dt_binding_check, add a DTB_FILES
variable that can be passed to the dtbs_check make command to restrict
which devicetrees are validated.
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@xxxxxxxxxxxxx>