Apple SoC MAINTAINERS updates for 5.20.

Just one patch adding our shiny new mailing list to the relevant
section.

* tag 'asahi-soc-maintainers-5.20' of https://github.com/AsahiLinux/linux:
  MAINTAINERS: add ARM/APPLE MACHINE mailing list

Link: https://lore.kernel.org/r/308defdf-8873-63ed-dc9c-a88cd7a3cc2f@marcan.stSigned-off-by: Arnd Bergmann <arnd@arndb.de>