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;
};