hikey: make console configurable

Signed-off-by: Victor Chong <victor.chong@linaro.org>
Tested-by: Victor Chong <victor.chong@linaro.org> (hikey)
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>