drivers: entropy: stm32 driver waits for the CondRst reset

For the stm32 mcu with a CONDRST bit in its RNG_CR
for conditioning Soft Reset, the driver must wait for
the bit to be 0 after disabling. This could take
about 2 AHB clock cycles + 2RNG clock cycles.

Signed-off-by: Francois Ramu <francois.ramu@st.com>