soc: mediatek: avoid hardcoded value with bus_prot_mask

use a meaningful definition for bus_prot_mask instead of just hardcoded
for it.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>