MAINTAINERS: Add an entry for the CHRP NVRAM files

I recently added new files to the source tree that are not
covered by any maintainer yet -- and since every new source
file should have a maintainer nowadays, I volunteer to look
after these files now, too.

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>