dt-bindings: mfd: Add A33 GPADC binding

This patch adds documentation for the A33 GPADC binding.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>