spi: Add Amlogic SPISG driver

Introduced support for the new SPI IP (SPISG) driver. The SPISG is
a communication-oriented SPI controller from Amlogic,supporting
three operation modes: PIO, block DMA, and scatter-gather DMA.

Due to there is no FIFO, PIO mode can only transfer one word at
a time, which is extremely slow. Therefore, this mode was not
implemented.

Signed-off-by: Sunny Luo <sunny.luo@amlogic.com>
Signed-off-by: Xianwei Zhao <xianwei.zhao@amlogic.com>
Link: https://patch.msgid.link/20250718-spisg-v5-2-b8f0f1eb93a2@amlogic.com


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