dt-bindings: mfd: ab8500: Document AB8505 bindings

AB8505 can now be configured from the device tree.
The configuration is almost identical to AB8500, so just add a note
for the nodes/compatibles that differ between the two revisions.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>