dt-bindings: mfd: Add Altera Arria10 SR Reset Controller bindings

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

Signed-off-by: Thor Thayer <thor.thayer@linux.intel.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>