fix(zynqmp): remove unused PLAT_NUM_POWER_DOMAINS

Remove unused PLAT_NUM_POWER_DOMAINS macro. Macro is referenced by
docs/design/psci-pd-tree.rst but it is not used in any calculation
that's why it is better to remove it.

Change-Id: I33f26cda6a4404061af5598ea4c751f64127e50a
Signed-off-by: Michal Simek <michal.simek@amd.com>
diff --git a/plat/xilinx/zynqmp/include/platform_def.h b/plat/xilinx/zynqmp/include/platform_def.h
index fb1130f..99a24ad 100644
--- a/plat/xilinx/zynqmp/include/platform_def.h
+++ b/plat/xilinx/zynqmp/include/platform_def.h
@@ -26,7 +26,6 @@
 #endif
 
 #define PLATFORM_CORE_COUNT		U(4)
-#define PLAT_NUM_POWER_DOMAINS		U(5)
 #define PLAT_MAX_PWR_LVL		U(1)
 #define PLAT_MAX_RET_STATE		U(1)
 #define PLAT_MAX_OFF_STATE		U(2)