binman: Allow entry types to override FDT contents

binman: Allow entry types to override FDT contents

At present the contents of an FDT (for each phase) are fixed,
determined by the build and provided to Binman as input files.

Provide a means for entry types to provide their own FDT, so that it can
be processed, if needed.

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