configs: add support of OPTEE RNG in stm32mp15 defconfig

When the RNG device is secured with OP-TEE, it is only accessible with
the HWRNG TA, the CONFIG_RNG_OPTEE is needed for STM32MP15 targets
with OP-TEE support.

The probe of this RNG driver fails when the TA is not available in OP-TEE
and the previous driver can be used, as CONFIG_RNG_STM32MP1 is activated
and when the associated node is activated in the device tree with:

&rng1 {
	status = "okay";
};

When the RNG is used in OP-TEE, this node should be deactivated in
the Linux and U-Boot device tree.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>