platform: Remove deprecated platform nRF5340 PDK

Remove the deprecated platform nRF5340 PDK
(nordic_nrf/nrf5340pdk_nrf5340_cpuapp)
from platform codebase and documentation.

Update also a few links for nRF5340 DK that were still
referring to nRF5340 PDK.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>
Change-Id: Ib39863b0b93b3c105d640e38e361a03a729d3052
diff --git a/docs/technical_references/tfm_secure_boot.rst b/docs/technical_references/tfm_secure_boot.rst
index 95d3f68..087a2dc 100644
--- a/docs/technical_references/tfm_secure_boot.rst
+++ b/docs/technical_references/tfm_secure_boot.rst
@@ -254,7 +254,7 @@
 +---------------------+-----------------+---------------+----------+----------------+--------------+
 | nRF9160 DK          | Yes             | Yes           | No       | No             | No           |
 +---------------------+-----------------+---------------+----------+----------------+--------------+
-| nRF5340 PDK/DK      | Yes             | Yes           | No       | No             | No           |
+| nRF5340 DK          | Yes             | Yes           | No       | No             | No           |
 +---------------------+-----------------+---------------+----------+----------------+--------------+
 
 .. [1] To disable BL2, please set the ``BL2`` cmake option to ``OFF``