dt-bindings: Add bindings for Azoteq IQS620A/621/622/624/625

This patch adds device tree bindings for the Azoteq IQS620A, IQS621,
IQS622, IQS624 and IQS625 multi-function sensors.

A total of three bindings are presented (one MFD and two child nodes);
they are submitted as a single patch because the child node bindings
have no meaning in the absence of the MFD binding.

Signed-off-by: Jeff LaBundy <jeff@labundy.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>