hwrng: optee-rng: use tee_shm_alloc_kernel_buf()

Uses the new simplified tee_shm_alloc_kernel_buf() function instead of
the old deprecated tee_shm_alloc() function which required specific
TEE_SHM-flags.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>