ACPI: Update information in MAINTAINERS

Because Rui is now going to focus on work that is not related to the
maintenance of kernel code, drop the MAINTAINERS records for the
ACPI fan and video drivers that will be maintained by Rafael along
with the rest of the ACPI subsystem.

While at it, change the information regarding the Len Brown's role in
the ACPI subsystem to "reviewer" to reflect the current status.

Signed-off-by: Rafael J. Wysocki <rafael@kernel.org>
Reviewed-by: Len Brown <len.brown@intel.com>