spi: dt-bindings: sifive: Add missing 2nd register region

The 'reg' description and example have a 2nd register region for memory
mapped flash, but the schema says there is only 1 region. Fix this.

Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: linux-spi@vger.kernel.org
Cc: linux-riscv@lists.infradead.org
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/spi/spi-sifive.yaml

@@ -32,11 +32,10 @@ properties:
       https://github.com/sifive/sifive-blocks/tree/master/src/main/scala/devices/spi
 
   reg:
-    maxItems: 1
-
-    description:
-      Physical base address and size of SPI registers map
-      A second (optional) range can indicate memory mapped flash
+    minItems: 1
+    items:
+      - description: SPI registers region
+      - description: Memory mapped flash region
 
   interrupts:
     maxItems: 1