I'm wonder the uart/usb converter based on CP210x will works with Arch linux distribution? or maybe there is an option to use onboard UART to communicate with my external device?
Yes, and yes.
The CP210x is supported, but I don't think the driver is included "out of the box". Rather, you'd need to compile your own kernel (or the relevant modules).
The second iMX233 "application" serial port is available for use - /dev/ttySP1 on 2.6 kernels, and /dev/ttyAPP1 on 3.x kernels.