arm: mvebu: serial: Add me as co-maintainer and author of Marvell serial drivers

There is no maintainer entry for serial_mvebu_a3700.c. Add entry with Pali
and Stefan as maintainers.

Signed-off-by: Pali Rohár <pali@kernel.org>
Acked-by: Stefan Roese <sr@denx.de>
Reviewed-by: Stefan Roese <sr@denx.de>