soc: arm: stm32: configure LDO supply on H7 init

Configure the LDO supply when initializing H7 (M7) SoC. I have
observed MCU hanging on LL_PWR_IsActiveFlag_VOS() wait loop when
doing a cold boot if LDO supply is not explicitely enabled.
According to the datasheet LDO should be enabled by default,
however HAL examples also configure LDO, so there may be a
reason to perform such step.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>