boards: arm: stm32h735g_disco: fix support for RNG

This board's documentation states the current configuration supports RNG
However, the RNG peripheral, and the HSI48 clock that the RNG depends
on, were not enabled

This commit enables the RNG and the HSI48 clock.

Verified with entropy test.

Signed-off-by: Ben Marsh <ben.marsh@helvar.com>