drivers: entropy: stm32: RCC_HSI48_SUPPORT not always defined

RCC_HSI48_SUPPORT is not always defined even if SOC support HSI48.
Ex: STM32H7 family which support HSI48 but doesn't have
switch RCC_HSI48_SUPPORT.
This switch is usefull when in the same STM32 family some soc have
HSI487 and some other soc not.

So instead, use CMSIS register defines (which depends on family)

Signed-off-by: Alexandre Bourdiol <alexandre.bourdiol@st.com>