board: stemmy: Update documentation

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>