Remove the prototypes for zoom2_serial_deviceN from serial.h . This
can't be done right away, as they are referenced from the zoom2
config file. Therefore, adjust the code so the config file only
specifies number of the port. Then, replace the simple return in
default_serial_console() with a switch across possible values, which
returns the zoom2_serial_deviceN . With such adjustment in place,
the exported prototypes in serial.h can be safely removed.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>