zynqmp: Enable workaround for errata 855873

Zynqmp implements a version of the Cortex A53 affected by errata 855873.
Enable the workaround for the errata and silence the warning: "WARNING:
BL31: cortex_a53: errata workaround for 855873 was missing!".

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>