dt-bindings: mfd: Add Altera Arria10 System Resource Chip bindings

The Altera Arria10 Devkit System Resource chip is a Multi-Function
Device with a GPIO expander.

This patch adds documentation for the Altera A10-SR DT bindings.

Signed-off-by: Thor Thayer <tthayer@opensource.altera.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>