doc: dt: add documentation for alphascale,asm9260-rtc

* Alphascale asm9260 SoC Real Time Clock

Required properties:

- compatible: Should be "alphascale,asm9260-rtc"

- reg: Physical base address of the controller and length

       of memory mapped region.

- interrupts: IRQ line for the RTC.

- clocks: Reference to the clock entry.

- clock-names: should contain:

  * "ahb" for the SoC RTC clock

Example:

rtc0: rtc@800a0000 {

	compatible = "alphascale,asm9260-rtc";

	reg = <0x800a0000 0x100>;

	clocks = <&acc CLKID_AHB_RTC>;

	clock-names = "ahb";

	interrupts = <2>;

};