mfd: madera: Add DT bindings for Cirrus Logic Madera codecs

Specification of the bindings for the parent MFD driver component
of the Cirrus Logic Madera codec drivers.

Note that although the interrupt controller and GPIO are child
drivers their required bindings are trivial, mandatory, and exist
within the parent MFD node so are documented here.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>