serial: mpc8xx: Move serial registration from serial_initialize()

serial: mpc8xx: Move serial registration from serial_initialize()

Move the registration of serial_smc and serial_scc ports from
default serial_initialize() into CPU 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 exports of both serial_scc_device and serial_smc_device can
not yet be removed as they are still used in default_serial_console()
calls.

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>