regmap: Add the DT binding documentation for endianness

Device-Tree binding for regmap

The endianness mode of CPU & Device scenarios:

Index     Device     Endianness properties

---------------------------------------------------

1         BE         'big-endian'

2         LE         'little-endian'

For one device driver, which will run in different scenarios above

on different SoCs using the devicetree, we need one way to simplify

this.

Required properties:

- {big,little}-endian: these are boolean properties, if absent

  meaning that the CPU and the Device are in the same endianness mode,

  these properties are for register values and all the buffers only.

Examples:

Scenario 1 : CPU in LE mode & device in LE mode.

dev: dev@40031000 {

	      compatible = "name";

	      reg = <0x40031000 0x1000>;

	      ...

};

Scenario 2 : CPU in LE mode & device in BE mode.

dev: dev@40031000 {

	      compatible = "name";

	      reg = <0x40031000 0x1000>;

	      ...

	      big-endian;

};

Scenario 3 : CPU in BE mode & device in BE mode.

dev: dev@40031000 {

	      compatible = "name";

	      reg = <0x40031000 0x1000>;

	      ...

};

Scenario 4 : CPU in BE mode & device in LE mode.

dev: dev@40031000 {

	      compatible = "name";

	      reg = <0x40031000 0x1000>;

	      ...

	      little-endian;

};