configs: stm32mp15_basic: Set regulator relative flags

Enable DM_REGULATOR_STPMU1 flag to activate regulator
driver for STM32MP15 SoC and CMD_REGULATOR flag to be
able to set/get regulator state int U-boot command line.

Disable PMIC_CHILDREN as this flag is not needed in SPL
for STM32MP1.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>