Documentation: Describe the AB8500 Device Tree bindings

Although for the most part, the AB8500 uses common bindings, some
of the ways in which they are used differ slightly to the common
uses of those bindings. To clear up some of these varying concepts
we provide some documentation describing each of the properties and
how they are used.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>