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Device-Tree binding for regmap

The endianness mode of CPU & Device scenarios:
Index     Device     Endianness properties
---------------------------------------------------
1         BE         'big-endian'
2         LE         'little-endian'
3	  Native     'native-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.

Optional properties:
- {big,little,native}-endian: these are boolean properties, if absent
  then the implementation will choose a default based on the device
  being controlled.  These properties are for register values and all
  the buffers only.  Native endian means that the CPU and device have
  the same endianness.

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;
};