MAINTAINERS: update file entry for HSI subsystem

The HSI documentation was moved into Documentation/driver-api/hsi.rst in
commit 5e995786 ("docs: split up serial-interfaces.rst"). Update the
corresponding file entry in MAINTAINERS.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>