Add support for a triple serial port configuration (USB_TRIPLE_SERIAL),
providing a composite USB device, comprised of three serial ports.
The third serial port is called usb_serial3 (C) or SerialB (C++).

Note that no dummy C++ class is created if USB_DISABLED is defined,
unlike for the first port.

This increases binary size by ca. 0.5 KiB (despite needing 720 more
bytes for USB buffers, as gcc-5.4.1 no longer decides to unroll a loop
over all endpoints in usb_isr()).

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

Add support for a dual serial port configuration (USB_DUAL_SERIAL),
providing a composite USB device, comprised of two serial ports.
The second serial port is called usb_serial2 (C) or SerialA (C++).

Note that no dummy C++ class is created if USB_DISABLED is defined,
unlike for the first port.

This increases binary size by ca. 1.2 KiB (720 bytes for USB buffers).

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>