MAINTAINERS: add myself as a maintainer for the sl28cpld

The sl28cpld is a management controller found on the Kontron SMARC-sAL28
board for now. Support for it was added by me quite a while ago, but I
didn't add a MAINTAINERS entry. Add it now.

Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20220207184652.1218447-1-michael@walle.cc

'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>