Digital 13 / SCK Digital I/O, or SPI clock
Digital 12 / MISO Digital I/O, or SPI receive data
Digital ~11 / MOSI Digital I/O, SPI send data, or PWM (configured with PWM swizzler)
Digital ~10 Digital I/O, SPI signal select, or PWM (configured with PWM swizzler)
So when using the mraa library, how do i define another cs pin? can it be any digital pin? it looks like the library already has the pin assignments built in?
You can use another GPIO to interact as a Cspin.
Also, take a look at the Hardware Guides for more information about the pins.