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>

plat/mediatek/mt8173/platform.mk

@@ -90,6 +90,7 @@ $(eval $(call add_define,ARM_GIC_ARCH))
 # Enable workarounds for selected Cortex-A53 erratas.
 ERRATA_A53_826319	:=	1
 ERRATA_A53_836870	:=	1
+ERRATA_A53_855873	:=	1
 
 # indicate the reset vector address can be programmed
 PROGRAMMABLE_RESET_ADDRESS	:=	1