spi: dt-bindings: Add binding document of Amlogic SPISG controller

The SPISG is a new communication oriented SPI controller of Amlogic, which
supports PIO, block DMA and scatter-gather DMA three operation modes.

Signed-off-by: Sunny Luo <sunny.luo@amlogic.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Xianwei Zhao <xianwei.zhao@amlogic.com>
Link: https://patch.msgid.link/20250718-spisg-v5-1-b8f0f1eb93a2@amlogic.com


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