configs: stm32mp15: Enable STM32_VREFBUF flag

Enable vrefbuf on stm32mp15, to be used by ADC.
Signed-off-by: NFabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: NPatrice Chotard <patrice.chotard@st.com>

configs/stm32mp15_basic_defconfig

@@ -36,6 +36,7 @@ CONFIG_DM_PMIC=y
 CONFIG_PMIC_STPMU1=y
 CONFIG_DM_REGULATOR=y
 CONFIG_DM_REGULATOR_GPIO=y
+CONFIG_DM_REGULATOR_STM32_VREFBUF=y
 CONFIG_DM_REGULATOR_STPMU1=y
 CONFIG_STM32_SERIAL=y
 # CONFIG_EFI_LOADER is not set