Add DMA support for RZ/T2H RSPI

Merge series from Cosmin Tanislav <cosmin-gabriel.tanislav.xa@renesas.com>:

The DMA controller can be used to transfer data to and from the SPI
controller without involving the CPU for each word of a SPI transfer.

Add support for DMA mode, and do some other cleanups while touching the
same code.

Author: broonie

Documentation/devicetree/bindings/spi/renesas,rzv2h-rspi.yaml

@@ -57,6 +57,14 @@ properties:
       - const: presetn
       - const: tresetn
 
+  dmas:
+    maxItems: 2
+
+  dma-names:
+    items:
+      - const: rx
+      - const: tx
+
   power-domains:
     maxItems: 1