MAINTAINERS: Make Daniel P. Smith sole XSM maintainer

MAINTAINERS: Make Daniel P. Smith sole XSM maintainer

While mail hasn't been bouncing, Daniel De Graaf has not been
responding to patch submissions or otherwise interacting with the
community for several years.  Daniel Smith has at least been working
with the code, and is a regular member of our community; and he has
agreed to step up into the role.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>