scripts/dtc: Update to upstream version v1.4.6-21-g84e414b0b5bc

This adds the following commits from upstream:

84e414b0b5bc tests: Add a test case for the omit-if-no-ref keyword
4038fd90056e dtc: add ability to make nodes conditional on them being referenced
e1f139ea4900 checks: drop warning for missing PCI bridge bus-range
f4eba68d89ee checks: Print duplicate node name instead of parent name
46df1fb1b211 .travis.yml: Run valgrind checks via Travis
14a3002a1aee tests: Update valgrind suppressions for sw_tree1
02c5fe9debc0 tests: Remove valgrind error from tests/get_path
df536831d02c checks: add graph binding checks
2347c96edcbe checks: add a check for duplicate unit-addresses of child nodes
8f1b35f88395 Correct overlay syntactic sugar for generating target-path fragments
afbddcd418fb Suppress warnings on overlay fragments
119e27300359 Improve tests for dtc overlay generation

Signed-off-by: Rob Herring <robh@kernel.org>