dts: bindings: spi: Add memory-region to nrf-spis

Add memory-region to nordic,nrf-spis binding. This commit aligns SPIS
shim to utilize memory-region property. This optional property enables
user to specify placement of dma buffers in memory region. It is done
by assigning to memory-region property, phandle to node with
zephyr,memory-region and mimo-sram compatible.

When memory-region property is not specified for given device, buffer
is placed in default RAM region with other data.

Signed-off-by: Krzysztof Chruściński <[email protected]>

dts/bindings/spi/nordic,nrf-spis.yaml

@@ -5,7 +5,7 @@ description: Nordic nRF family SPIS (SPI slave with EasyDMA)
 
 compatible: "nordic,nrf-spis"
 
-include: nordic,nrf-spi-common.yaml
+include: ["nordic,nrf-spi-common.yaml", "memory-region.yaml"]
 
 properties:
   def-char: