* now uses new subsystem framework

Add support for more PCIe based devices

See merge request !19

Move driver init code into PCIe probe

See merge request !18

	- All pcie boards now use a MMCM to generate design clocks
	- Tapasco Status is currently not compatible to US+ -> compilation fails

* must use platform_res_t and platform_strerror to decode error messages
  returned from the Platform API

* PLATFORM_SUCCESS was defined as 0, whereas TAPASCO_SUCCESS was defined
  as 1; on success, these values should be compatible with each other
* changed the definition of platform_res_t
* platform_strerror now returns "unknown error" for unknown error codes

    - Don't access the device if the device is not available
    - Properly free buffers if device becomes unavailable