drivers: entropy: stm32: Add support for STM32L0 SoCs

Add the necessary clock configuration to support STM32L-based
SoCs. This change likely adds support for other STM32 SoCs as well
since the HSI48 clock is configured for all SoCs that support it
(except the STM32L4x) instead of just the STM32G4X.

Signed-off-by: Andreas Sandberg <andreas@sandberg.pp.se>