hwrng: add OP-TEE based rng driver

On ARM SoC's with TrustZone enabled, peripherals like entropy sources
might not be accessible to normal world (linux in this case) and rather
accessible to secure world (OP-TEE in this case) only. So this driver
aims to provides a generic interface to OP-TEE based random number
generator service.

This driver registers on TEE bus to interact with OP-TEE based rng
device/service.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>