boards: custom_plank: suppress some DT warnings

Nordic boards have some overlapping DT addresses, silent dtc as
this is a known issue.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>

Author: gmarull

boards/vendor/custom_plank/pre_dt_board.cmake

@@ -0,0 +1,7 @@
+# Copyright (c) 2024 Nordic Semiconductor
+# SPDX-License-Identifier: Apache-2.0
+
+# Suppress "unique_unit_address_if_enabled" to handle the following overlaps:
+# - power@40000000 & clock@40000000 & bprot@40000000
+# - acl@4001e000 & flash-controller@4001e000
+list(APPEND EXTRA_DTC_FLAGS "-Wno-unique_unit_address_if_enabled")