regulator: dt-bindings: samsung,s5m8767: correct s5m8767,pmic-buck-default-dvs-idx property

The driver was always parsing "s5m8767,pmic-buck-default-dvs-idx", not
"s5m8767,pmic-buck234-default-dvs-idx".

Cc: <stable@vger.kernel.org>
Fixes: 26aec009 ("regulator: add device tree support for s5m8767")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Acked-by: Rob Herring <robh@kernel.org>
Message-Id: <20211008113723.134648-3-krzysztof.kozlowski@canonical.com>
Signed-off-by: Mark Brown <broonie@kernel.org>