Documentation: arm: remove stih415/stih416 related entries

ST's STiH415 and STiH416 platforms support have been removed since
a long time already.  This commit updates the sti related documentation
overview to remove related entries and update the sti part to add
STiH407/STiH410 and STiH418 platforms which are still actively
supported.

Signed-off-by: Alain Volmat <avolmat@me.com>
Link: https://lore.kernel.org/r/20230416185349.18156-1-avolmat@me.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>