arm64: dts: cix: add a compatible string for the cix sky1 SoC

arm64: dts: cix: add a compatible string for the cix sky1 SoC

The SPI IP design for the cix sky1 SoC uses a FIFO with a data width
of 32 bits, instead of the default 8 bits. Therefore, a compatible
string is added to specify the FIFO data width configuration for the
cix sky1 SoC.

Signed-off-by: Jun Guo <jun.guo@cixtech.com>
Link: https://lore.kernel.org/r/20251031073003.3289573-4-jun.guo@cixtech.com
Signed-off-by: Peter Chen <peter.chen@cixtech.com>