With a few GPIO pins, your system can bitbang the SPI protocol.
Select this to get SPI support through I/O pins (GPIO, parallel
port, etc). Or, some systems' SPI master controller drivers use
this code to manage the per-word or per-transfer accesses to the
hardware shift registers.

This is library code, and is automatically selected by drivers that
need it. You only need to select this explicitly to support driver
modules that aren't part of this kernel tree.

source code:

drivers/spi/spi-bitbang.c