Move the registration of s5p_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 s5p_serial_initialize() function, which is
implemented inside of the serial_s5p driver allows encapsulation
of s5p_serialN_device within the serial_s5p driver itself.

Also, remove the exports of s5p_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_s5p 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: Minkyu Kang <mk7.kang@samsung.com>