MAINTAINERS: update drivers/platform/x86 information

MAINTAINERS: update drivers/platform/x86 information

Many of the drivers/platform/x86 drivers have nothing to do with ACPI, so
it's kind of inappropriate for them to be stuck under the ACPI mailing
list. Add a new mailing list (platform-driver-x86@vger.kernel.org) and,
with Len's blessing, add myself as subsystem maintainer.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Cc: Anisse Astier <anisse@astier.eu>
Cc: Carlos Corbacho <carlos@strangeworlds.co.uk>
Cc: Cezary Jackiewicz <cezary.jackiewicz@gmail.com>
Cc: Corentin Chary <corentincj@iksaif.net>
Cc: Daniel Oliveira Nascimento <don@syst.com.br>
Cc: Harald Welte <laforge@gnumonks.org>
Cc: Henrique de Moraes Holschuh <ibm-acpi@hmh.eng.br>
Cc: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
Cc: Jonathan Woithe <jwoithe@physics.adelaide.edu.au>
Cc: Karol Kozimor <sziwan@users.sourceforge.net>
Cc: Len Brown <lenb@kernel.org>
Cc: Lennart Poettering <mzxreary@0pointer.de>
Cc: Mattia Dongili <malattia@linux.it>
Cc: Peter Feuerer <peter@piie.net>
Cc: Sujith Thomas <sujith.thomas@intel.com>
Cc: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>