MAINTAINERS: Change mentions of mpm to olivia

Following this mercurial changeset:
https://www.mercurial-scm.org/repo/hg-stable/rev/d4ba4d51f85f

update the MAINTAINERS entry to replace the now obsolete identity.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220712185419.45487-1-f.fainelli@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>