board: stemmy: Update documentation

Over the time, the "stemmy" U-Boot board was tested on several other
Samsung smartphones based on ST-Ericsson NovaThor Ux500. Convert the
documentation to reStructuredText at doc/board/ste/stemmy.rst and
make the device list complete. Also note that the board now boots
into USB Fastboot instead of just ending up at the U-Boot prompt.

The device table is mostly taken from the postmarketOS wiki article
(https://wiki.postmarketos.org/wiki/ST-Ericsson_NovaThor_U8500).
All the newly added devices were tested by Linus Walleij.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
diff --git a/doc/board/ste/index.rst b/doc/board/ste/index.rst
new file mode 100644
index 0000000..bef520c
--- /dev/null
+++ b/doc/board/ste/index.rst
@@ -0,0 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+ST-Ericsson
+===========
+
+.. toctree::
+   :maxdepth: 2
+
+   stemmy