Rework i.MX53 framebuffer driver

* Simplify IPU register definitions using templates
* Distinguish between i.MX53 QSB and SMD board in driver
* Support IPU specific overlay mechanism by framebuffer session extension
Double buffering for i.MX53 fb driver (fix #721)

Enable optinal support for double buffering in the i.MX53 framebuffer
driver. This prevents flickering in certain scenarios, where applications
directly render in the framebuffer dataspace given by the driver.