MAINTAINERS: add myself as co-maintainer for Realtek DSA switch drivers

Adding myself (Alvin Šipraga) as another maintainer for the Realtek DSA
switch drivers. I intend to help Linus out with reviewing and testing
changes to these drivers, particularly the rtl8365mb driver which I
authored and have hardware access to.

Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>