MAINTAINERS: Add FSI subsystem

The subsystem was merged some time ago but we did not have a maintainers
entry. The mailing list exists to allow our patchwork to slurp up the
patches. The tree will be co-maintained by myself and Jeremy.

Acked-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>