documentation: dt: mct: add desc of optional property use-cp15-phys-counter

index 167d5da..c7f6354 100644
@@ -36,6 +36,16 @@ Required properties:
   interrupt might be specified, meaning that all local timers use the same
   per processor interrupt.
 
+Optional properties:
+
+- use-cp15-phys-counter : set to advise mct clocksource driver to use ARM
+  arch timer 64-bit counter as main counter. Access to this coprocessor
+  counter is faster than access to same counter in mct mmio region.
+  It was discovered that mct and ARM arch timer are same underlying hardware
+  on some SoCs.
+  Only supported for Exynos5-based 32-bit systems which follow the ARMv7
+  architecture.
+
 Example 1: In this example, the IP contains two local timers, using separate
 	   interrupts, so two local timer interrupts have been specified,
 	   in addition to four global timer interrupts.