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>

plat/xilinx/zynqmp/platform.mk

@@ -27,6 +27,7 @@
 # ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 # POSSIBILITY OF SUCH DAMAGE.
 
+override ERRATA_A53_855873 := 1
 override ENABLE_PLAT_COMPAT := 0
 override PROGRAMMABLE_RESET_ADDRESS := 1
 PSCI_EXTENDED_STATE_ID := 1