MAINTAINERS: Move MFD from a Supported to Maintaied state

No one is funding MFD maintenance.

  S: *Status*, one of the following:
     Supported:   Someone is actually paid to look after this.
     Maintained:  Someone actually looks after it.

Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221206105038.124613-1-lee@kernel.org