binman: Update fit to use node instead of subnode

It doesn't make sense to use 'subnode' as a function parameter since it
is just a 'node' so far as the function is concerned. Update two functions
to use 'node' instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>