spi: spi-kspi2: Add KEBA SPI controller support

The KEBA SPI controller is found in the system FPGA of KEBA PLC devices.
It is used to connect the SPI flash chip of the FPGA and some SPI
devices.

It is a simple SPI controller with configurable speed. The hardware
supports only single byte transfers. There are no FIFOs or interrupts.

Signed-off-by: Gerhard Engleder <eg@keba.com>
Link: https://patch.msgid.link/20241202194003.57679-1-gerhard@engleder-embedded.com


Signed-off-by: Mark Brown <broonie@kernel.org>