boards: riscv: neorv32: mention TRNG support
boards: riscv: neorv32: mention TRNG support

Mention TRNG support in the NEORV32 board documentation, but leave it
disabled by default.

Signed-off-by: Henrik Brix Andersen <a076b11c2cc63ccdc903dbe182e1eecc3cbbc094@brixandersen.dk>

galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr