support for amlogic the new SPI IP

Merge series from Xianwei Zhao <xianwei.zhao@amlogic.com>:

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

Add the drivers and device tree bindings corresponding to the SPISG.