plat/mediatek: Enable Cortex-A53 erratum 855873 workaround

The Mediatek 8173 SoC contains Cortex-A53 CPUs which are affected by
erratum 855873.

Enable the workaround that TF provides to fix this erratum.

Change-Id: I6e1c7822c320d81bdd46b8942d1d755883dac1f5
Signed-off-by: Andre Przywara <andre.przywara@arm.com>