drivers: entropy: Clock activation is missing on stm32l4

On STM32L4 SoCs RNG is clocked by 48MHz domain.
Hence, besides clock activation, it requires 48M
domain to be enabled.

Tested on:
*nucleo_l476rg
*stm32l476g_disco
*disco_l475_iot1

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>