Move the registration of serial_XXuart 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 pxa_serial_initialize() function, which is
implemented inside of the serial_pxa driver allows encapsulation
of serial_XXuart within the serial_pxa driver itself.

Also, remove the exports of serial_XXuart 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_pxa driver and
the default console is picked by CONFIG_CONS_IDX 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>