serial: ns16550: Move serial registration from serial_initialize()

Move the registration of eserialN_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 ns16550_serial_initialize() function, which is
implemented inside of the ns16550 serial driver allows encapsulation
of eserialN_device within the ns16550 serial driver itself.

Also, remove the exports of eserialN_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 ns16550 serial driver and
the default console is picked by CONFIG_SERIAL<N> macro in config file.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Michal Simek <monstr@monstr.eu>