dt-bindings: riscv: correct e51 and u54-mc CPU bindings

All existing boards with sifive,e51 and sifive,u54-mc use it on top of
sifive,rocket0 compatible:

  arch/riscv/boot/dts/microchip/microchip-mpfs-icicle-kit.dt.yaml: cpu@0: compatible: 'oneOf' conditional failed, one must be fixed:
    ['sifive,e51', 'sifive,rocket0', 'riscv'] is too long
    Additional items are not allowed ('riscv' was unexpected)
    Additional items are not allowed ('sifive,rocket0', 'riscv' were unexpected)
    'riscv' was expected
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210920132559.151678-1-krzysztof.kozlowski@canonical.comSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/riscv/cpus.yaml

@@ -31,9 +31,7 @@ properties:
               - sifive,bullet0
               - sifive,e5
               - sifive,e7
-              - sifive,e51
               - sifive,e71
-              - sifive,u54-mc
               - sifive,u74-mc
               - sifive,u54
               - sifive,u74
@@ -41,6 +39,12 @@ properties:
               - sifive,u7
               - canaan,k210
           - const: riscv
+      - items:
+          - enum:
+              - sifive,e51
+              - sifive,u54-mc
+          - const: sifive,rocket0
+          - const: riscv
       - const: riscv    # Simulator only
     description:
       Identifies that the hart uses the RISC-V instruction set