dt-bindings: clk: Add bindings for Fixed MMIO clock

This patch adds a DT binding documentation for Fixed
Memory Mapped IO clocks.
Signed-off-by: Jan Kotas <jank@cadence.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

Documentation/devicetree/bindings/clock/fixed-mmio-clock.txt

+Binding for simple memory mapped io fixed-rate clock sources.
+The driver reads a clock frequency value from a single 32-bit memory mapped
+I/O register and registers it as a fixed rate clock.
+
+It was designed for test systems, like FPGA, not for complete, finished SoCs.
+
+This binding uses the common clock binding[1].
+
+[1] Documentation/devicetree/bindings/clock/clock-bindings.txt
+
+Required properties:
+- compatible : shall be "fixed-mmio-clock".
+- #clock-cells : from common clock binding; shall be set to 0.
+- reg : Address and length of the clock value register set.
+
+Optional properties:
+- clock-output-names : From common clock binding.
+
+Example:
+sysclock: sysclock@fd020004 {
+	#clock-cells = <0>;
+	compatible = "fixed-mmio-clock";
+	reg = <0xfd020004 0x4>;
+};