configs: default activate CONFIG_RNG_STM32 for STM32MP13x platforms

configs: default activate CONFIG_RNG_STM32 for STM32MP13x platforms

Default embed this configuration. If OP-TEE PTA RNG is exposed, it means
that the RNG is managed by the secure world. Therefore, the RNG node
should be disabled in the device tree as an access would be denied
by the hardware firewall.

Signed-off-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>