regmap: Add the DT binding documentation for endianness

Device-Tree binding for device endianness
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.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>