dt-bindings: power: renesas,apmu: Fix cpus property limits

"make dtbs_check":

    arch/arm/boot/dts/r8a7791-koelsch.dtb: apmu@e6152000: cpus:0: [6, 7] is too long
	    From schema: Documentation/devicetree/bindings/power/renesas,apmu.yaml

Correct the minimum and maximum number of CPUs controlled by a single
APMU instance.

Fixes: 39bd2b6a ("dt-bindings: Improve phandle-array schemas")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/9ece1a07bbcb95abc9d80e6a6ecc95806a294a11.1648645279.git.geert+renesas@glider.be

Documentation/devicetree/bindings/power/renesas,apmu.yaml

@@ -36,7 +36,8 @@ properties:
   cpus:
     $ref: /schemas/types.yaml#/definitions/phandle-array
     items:
-      maxItems: 1
+      minItems: 1
+      maxItems: 4
     description: |
       Array of phandles pointing to CPU cores, which should match the order of
       CPU cores used by the WUPCR and PSTR registers in the Advanced Power