clocksource: armada-370-xp: Add detailed clock requirements in devicetree binding

Specifies the required clock inputs for each supported compatible.
Armada 370 requires a single clock phandle, and Armada XP requires
two clock phandles with clock-names "nbclk" and "fixed".

Cc: [e-mail]
Signed-off-by: Ezequiel Garcia <[e-mail]>
Signed-off-by: Daniel Lezcano <[e-mail]>
Acked-by: Jason Cooper <[e-mail]>
Acked-by: Stephen Warren <[e-mail]>
Acked-by: Gregory CLEMENT <[e-mail]>

Documentation/devicetree/bindings/timer/marvell,armada-370-xp-timer.txt

···
 - reg: Should contain location and length for timers register. First
   pair for the Global Timer registers, second pair for the
   local/private timers.
-- clocks: clock driving the timer hardware, only required for
-  "marvell,armada-370-timer";
+
+Clocks required for compatible = "marvell,armada-370-timer":
+- clocks : Must contain a single entry describing the clock input
+
+Clocks required for compatible = "marvell,armada-xp-timer":
+- clocks : Must contain an entry for each entry in clock-names.
+- clock-names : Must include the following entries:
+  "nbclk" (L2/coherency fabric clock),
+  "fixed" (Reference 25 MHz fixed-clock).
 
 Examples:
 
···
 		compatible = "marvell,armada-xp-timer";
 		reg = <0x20300 0x30>, <0x21040 0x30>;
 		interrupts = <37>, <38>, <39>, <40>, <5>, <6>;
+		clocks = <&coreclk 2>, <&refclk>;
+		clock-names = "nbclk", "fixed";
 	};