hikey*.mk: Support ptable-linux-8g.img build

Fixes: https://github.com/OP-TEE/build/issues/64
Fixes: https://github.com/OP-TEE/optee_os/issues/1296


Signed-off-by: Victor Chong <victor.chong@linaro.org>
Tested-by: Victor Chong <victor.chong@linaro.org> (build only)
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
[Update commit message: turn "Fixes X" into "Fixes: X" tags]
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>