soc: st: stm32wb0: support SoCs without SMPS output current limit

The STM32WB06 and STM32WB07 SoCs do not support SMPS output current limit.
This makes the LL_PWR_SetSMPSPrechargeLimitCurrent function and all the
LL_PWR_SMPS_PRECH_LIMIT_CUR_xxx defines not visible when one of these SoCs
is selected, resulting in a build failure.

Fix this by only handling SMPS current limit when the feature is available.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>