doc: dts: explain the usage of pre_dt_shield.cmake

Mention the possible use of pre_dt_shield.cmake in the devicetree and
shield documentation.

Signed-off-by: Paul Schaetzle <paulschaetzle@mailbox.org>

doc/build/dts/intro-input-output.rst

@@ -112,6 +112,9 @@ this:
 
    list(APPEND EXTRA_DTC_FLAGS "-Wno-simple_bus_reg")
 
+Shield directories can contain a file named :file:`pre_dt_shield.cmake` which
+has the same functionality as the aforementioned :file:`pre_dt_board.cmake`.
+
 .. _dt-outputs:
 
 Output files

doc/hardware/porting/shields.rst

@@ -19,7 +19,8 @@ under :zephyr_file:`boards/shields`:
    boards/shields/<shield>
    ├── <shield>.overlay
    ├── Kconfig.shield
-   └── Kconfig.defconfig
+   ├── Kconfig.defconfig
+   └── pre_dt_shield.cmake
 
 These files provides shield configuration as follows:
 
@@ -37,6 +38,9 @@ These files provides shield configuration as follows:
   shield configuration should be done by keeping in mind that features
   activation is application responsibility.
 
+* **pre_dt_shield.cmake**: This optional file can be used to pass additional
+  arguments to the devicetree compiler ``dtc``.
+
 Besides, in order to avoid name conflicts with devices that may be defined at
 board level, it is advised, specifically for shields devicetree descriptions,
 to provide a device nodelabel is the form <device>_<shield>, for instance: