doc: board: Add missing board index.rst files

These board index.rst files are missing in previous html doc patch
series, hence it causes some build warnings. Add those files.

Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

doc/board/AndesTech/index.rst

@@ -0,0 +1,10 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Andes Tech
+==========
+
+.. toctree::
+   :maxdepth: 2
+
+   adp-ag101p
+   ax25-ae350

doc/board/atmel/index.rst

@@ -0,0 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Atmel
+=====
+
+.. toctree::
+   :maxdepth: 2
+
+   at91ek

doc/board/freescale/index.rst

@@ -0,0 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Freescale
+=========
+
+.. toctree::
+   :maxdepth: 2
+
+   b4860qds

doc/board/renesas/index.rst

@@ -0,0 +1,10 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Renesas
+=======
+
+.. toctree::
+   :maxdepth: 2
+
+   sh7752evb
+   sh7753evb

doc/board/sifive/index.rst

@@ -0,0 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+SiFive
+======
+
+.. toctree::
+   :maxdepth: 2
+
+   fu540

doc/board/xilinx/index.rst

@@ -0,0 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Xilinx
+======
+
+.. toctree::
+   :maxdepth: 2
+
+   zynq