* ICP DAS LP-8841 SPI Controller for RTC

ICP DAS LP-8841 contains a DS-1302 RTC. RTC is connected to an IO
memory register, which acts as an SPI master device.

The device uses the standard MicroWire half-duplex transfer timing.
Master output is set on low clock and sensed by the RTC on the rising
edge. Master input is set by the RTC on the trailing edge and is sensed
by the master on low clock.

Required properties:

- #address-cells: should be 1

- #size-cells: should be 0

- compatible: should be "icpdas,lp8841-spi-rtc"

- reg: should provide IO memory address

Requirements to SPI slave nodes:

- There can be only one slave device.

- The spi slave node should claim the following flags which are
  required by the spi controller.

  - spi-3wire: The master itself has only 3 wire. It cannor work in
    full duplex mode.

  - spi-cs-high: DS-1302 has active high chip select line. The master
    doesn't support active low.

  - spi-lsb-first: DS-1302 requires least significant bit first
    transfers. The master only support this type of bit ordering.


Example:

spi@901c {
	#address-cells = <1>;
	#size-cells = <0>;
	compatible = "icpdas,lp8841-spi-rtc";
	reg = <0x901c 0x1>;

	rtc@0 {
		compatible = "maxim,ds1302";
		reg = <0>;
		spi-max-frequency = <500000>;
		spi-3wire;
		spi-lsb-first;
		spi-cs-high;
	};
};