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 <krzysztof.chruscinski@nordicsemi.no>