docs: Pull the HSI documentation together

The HSI subsystem documentation was split across hsi.txt and the
device-drivers docbook.  Now that the latter has been converted to Sphinx,
pull in the HSI document so that it's all in one place.

Acked-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>