devtree: add binding documentation for sp804

The sp804 binding is already in use by several platforms. This adds missing
documentation for the binding and also extends the binding to handle some
additional possible interrupt configurations.

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>