fdtgrep: Correct alignment of struct section

When outputting a devicetree we should not align the struct section to a
16-byte boundary. The normal position is fine, which is 8-byte aligned.

This avoids leaving adding 8 extra zero bytes in the output tree in the
case where the reserved section is empty (i.e has 16 zero bytes).

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