dt-bindings: rtc: Add on-chip ASPEED RTC documentation

Describe the RTC as used in the ASPEED BMC SoCs.
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/rtc/rtc-aspeed.txt

+ASPEED BMC RTC
+==============
+
+Required properties:
+ - compatible: should be one of the following
+   * aspeed,ast2400-rtc for the ast2400
+   * aspeed,ast2500-rtc for the ast2500
+   * aspeed,ast2600-rtc for the ast2600
+
+ - reg: physical base address of the controller and length of memory mapped
+   region
+
+ - interrupts: The interrupt number
+
+Example:
+
+   rtc@1e781000 {
+           compatible = "aspeed,ast2400-rtc";
+           reg = <0x1e781000 0x18>;
+           interrupts = <22>;
+           status = "disabled";
+   };