MAINTAINERS: add self for the dz serial driver

Now that I have got the necessary piece of hardware (thanks, Thiemo!), I may
well offer myself as the maintainer for the dz serial driver.  I hope nobody
objects.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>