I want to establish a communication between a Raspberry Pi and an Alchitry Cu. I found this and I wonder if any GPIO pins can be used for this, or they have to be dedicated pins. I mean, can I use any four pins for ss, miso, mosi and sck?
As with most things in the FPGA, any pins are useable. The only real exceptions to this are if you need special IO features like differential pairs or clock inputs. Clock inputs only need special pins if they are used to clock DFFs. The clock from SPI can easily be deciphered by DFFs clocked by your main system clock (as the example SPI peripheral does)