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

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";

};