ARM: dts: st: stm32mp157c-emstamp: drop incorrect vref_ddr property

The STPMIC1 PMIC vref_ddr regulator does not support over-current
protection, according to bindings and Linux driver:

  stm32mp157c-emsbc-argon.dtb: stpmic@33: regulators:vref_ddr: 'regulator-over-current-protection' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>