dt-bindings: spi: dw-apb-ssi: Add Canaan K210 SPI controller

Update the snps,dw-apb-ssi.yaml document to include the compatibility
string "canaan,k210-spi" compatible string for the Canaan Kendryte K210
RISC-V SoC DW apb_ssi V4 SPI controller.
Signed-off-by: NDamien Le Moal <damien.lemoal@wdc.com>
Acked-by: NSerge Semin <fancer.lancer@gmail.com>
Link: https://lore.kernel.org/r/20201206011817.11700-2-damien.lemoal@wdc.comSigned-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/snps,dw-apb-ssi.yaml

@@ -65,6 +65,8 @@ properties:
         const: baikal,bt1-ssi
       - description: Baikal-T1 System Boot SPI Controller
         const: baikal,bt1-sys-ssi
+      - description: Canaan Kendryte K210 SoS SPI Controller
+        const: canaan,k210-spi
 
   reg:
     minItems: 1