soc: arm: st_stm32: use SMPS power supply only if enabled

Use SMPS power supply only if enabled.

The default power supply configuration for the
NUCLEO board with -Q subfix is SMPS,
so it's essential to match with hardware configuration
to avoid deadlocks due to mismatch.

if a custom board with LDO configuration is in use,
then no need to enable `CONFIG_POWER_SUPPLY_SMPS`

Signed-off-by: Manojkumar Subramaniam <manoj@electrolance.com>