of: Document {little,big,native}-endian bindings

These apply to newly converted drivers, like serial8250/libahci/...
The examples were adapted from the regmap bindings document.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rob Herring <robh@kernel.org>