doc: dt: add documentation for nxp,lpc1788-rtc

Document NXP LPC178x/18xx/408x/43xx bindings

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

diff --git a/Documentation/devicetree/bindings/rtc/nxp,lpc1788-rtc.txt b/Documentation/devicetree/bindings/rtc/nxp,lpc1788-rtc.txt
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+NXP LPC1788 real-time clock
+
+The LPC1788 RTC provides calendar and clock functionality
+together with periodic tick and alarm interrupt support.
+
+Required properties:
+- compatible   : must contain "nxp,lpc1788-rtc"
+- reg          : Specifies base physical address and size of the registers.
+- interrupts   : A single interrupt specifier.
+- clocks       : Must contain clock specifiers for rtc and register clock
+- clock-names  : Must contain "rtc" and "reg"
+  See ../clocks/clock-bindings.txt for details.
+
+Example:
+rtc: rtc@40046000 {
+       compatible = "nxp,lpc1788-rtc";
+       reg = <0x40046000 0x1000>;
+       interrupts = <47>;
+       clocks = <&creg_clk 0>, <&ccu1 CLK_CPU_BUS>;
+       clock-names = "rtc", "reg";
+};