fdtget: Fix multiple arg bug and add test for it

There is a rather unfortunate bug in fdtget in that if multiple argument
sets are provided, it just repeats displaying the first set ones for
each set.

Fix this bug and add a test for it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: David Gibson <david@gibson.dropbear.id.au>