dt-bindings: perf: riscv,pmu: drop unneeded quotes

Drop unneeded quotes over simple string values to fix a soon to be
enabled yamllint warning:

  [error] string value is redundantly quoted with any quotes (quoted-strings)
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231122224414.2809184-1-robh@kernel.orgSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/devicetree/bindings/perf/riscv,pmu.yaml

@@ -90,7 +90,7 @@ properties:
             bitmap of all MHPMCOUNTERx that can monitor the range of events
 
 dependencies:
-  "riscv,event-to-mhpmevent": [ "riscv,event-to-mhpmcounters" ]
+  riscv,event-to-mhpmevent: [ "riscv,event-to-mhpmcounters" ]
 
 required:
   - compatible