fix(allwinner): improve DTB patching error handling

Currently, if any step of the DTB patching process fails, the whole
process is aborted. However, this causes some problems:
 - If any step modifies the DTB (including fdt_open_into), the dcache
   must still be cleaned, even if some later step fails.
 - The DTB may need changes in multiple places; if one patch fails (for
   example due to missing nodes), we should still apply other patches.
 - Similarly, if some patch fails, we should still run fdt_pack to
   clean up after ourselves.

Change-Id: If1af2e58e5a7edaf542354bb8a261dd1c3da1ad0
Signed-off-by: Samuel Holland <samuel@sholland.org>