dt-bindings: mailbox: qcom: Add clock-name optional property

When the APCS clock is registered (platform dependent), it retrieves
its parent names from hardcoded values in the driver.

The following commit allows the DT node to provide such clock names to
the platform data based clock driver therefore avoiding having to
explicitly embed those names in the clock driver source code.

Co-developed-by: Niklas Cassel <[e-mail]>
Signed-off-by: Niklas Cassel <[e-mail]>
Signed-off-by: Jorge Ramirez-Ortiz <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Reviewed-by: Bjorn Andersson <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Stephen Boyd <[e-mail]>

Documentation/devicetree/bindings/mailbox/qcom,apcs-kpss-global.txt

···
 	Usage: required
 	Value type: <prop-encoded-array>
 	Definition: must specify the base address and size of the global block
+
 - clocks:
-	Usage: required if #clocks-cells property is present
-	Value type: <phandle>
-	Definition: phandle to the input PLL, which feeds the APCS mux/divider
+	Usage: required if #clock-names property is present
+	Value type: <phandle array>
+	Definition: phandles to the two parent clocks of the clock driver.
 
 - #mbox-cells:
 	Usage: required
···
 	Value type: <u32>
 	Definition: as described in clock.txt, must be 0
 
+- clock-names:
+	Usage: required if the platform data based clock driver needs to
+	retrieve the parent clock names from device tree.
+	This will requires two mandatory clocks to be defined.
+	Value type: <string-array>
+	Definition: must be "pll" and "aux"
 
 = EXAMPLE
 The following example describes the APCS HMSS found in MSM8996 and part of the
···
 		reg = <0xb011000 0x1000>;
 		#mbox-cells = <1>;
 		clocks = <&a53pll>;
+		#clock-cells = <0>;
+	};
+
+Below is another example of the APCS binding on QCS404 platforms:
+
+	apcs_glb: mailbox@b011000 {
+		compatible = "qcom,qcs404-apcs-apps-global", "syscon";
+		reg = <0x0b011000 0x1000>;
+		#mbox-cells = <1>;
+		clocks = <&apcs_hfpll>, <&gcc GCC_GPLL0_AO_OUT_MAIN>;
+		clock-names = "pll", "aux";
 		#clock-cells = <0>;
 	};