serial: zynq: Move serial registration from serial_initialize()

Move the registration of zynq_serialN_device ports from default
serial_initialize() into driver specific function called from
serial_initialize(). This slims down the serial_initialize() call
to a bare tracker of all possible serial port registration routines
in U-Boot.

The newly implemented zynq_serial_initialize() function, which is
implemented inside of the serial_zynq driver allows encapsulation
of zynq_serialN_device within the serial_zynq driver itself.

Also, remove the exports of zynq_serialN_device from include/serial.h
as they are no longer needed. This is simply because the implementation of
default_serial_console() is wrapped into the serial_zynq driver and
the default console is picked by CONFIG_SERIAL<N> macro in config file.

Signed-off-by: Tom Rini <trini@ti.com>