tee: optee: take DT status property into account

DT nodes may have a 'status' property which, if set to anything other
than 'ok' or 'okay', indicates to the OS that the DT node should be
treated as if it was not present. So add that missing logic to the
OP-TEE driver.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>