arm: stm32: nucleo_g431rb: Enable support for Random Number Generator (RNG)

The nucleo_g431rb board is equipped with STM32G491RE microcontroller,
which do have the RNG IP block.

However, by default it is not enabled. To make it working properly, the
provided clock needs to fulfill following condition:
fRNGCLK < fHCLK/32, otherwise RNG_SR would set CEIS (bit5) and CECS
(bit1).
When combined with enabled interrupt from RNG, one would experience the
interrupt storm.

In this patch the DTS for this board has been adjusted properly, as well
as the board description has been updated to reflect RNG support.

Signed-off-by: Lukasz Majewski <lukma@denx.de>