ARM: kirkwood: reduce dependence of including platform file

For files like the drivers/serial/serial.c, it must include the
platform file, as the CONFIG_SYS_NS16550_COM1 must reference to the
definition in the platform definition files.

Include the platform definition file in the config file, so that it
would decouple the dependence for the driver files.

Signed-off-by: Lei Wen <leiwen@marvell.com>