MAINTAINERS: Mark simple firmware interface (SFI) obsolete

Len Brown has not been active in this part since around 2010 and
confirmed that he is not maintaining this part of the kernel sources
anymore and the git log suggests that nobody is actively maintaining it.

The referenced git tree does not exist. Instead, I found an sfi branch
in Len's kernel git repository, but that has not been updated since 2014;
so that is not worth to be mentioned in MAINTAINERS now anymore either.

Len Brown expects no further systems to be shipped with SFI, so we can
mark it obsolete and schedule it for deletion.

This change was motivated after I found that I could not send any mails
to the sfi-devel mailing list, and that the mailing list does not exist
anymore.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200118082545.23464-1-lukas.bulwahn@gmail.com