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.org


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>