firmware: xilinx: Add debugfs interface

Firmware-debug provides debugfs interface to all APIs.
Debugfs can be used to call firmware APIs with required
parameters.

Usage:
* Calling firmware API through debugfs:
  # echo "<api-name> <arg1> .. <argn>" > /sys/.../zynqmp-firmware/pm

* Read output of last called firmware API:
  # cat /sys/.../zynqmp-firmware/pm

Refer ug1200 for more information on these APIs:
  * https://www.xilinx.com/support/documentation/user_guides/ug1200-eemi-api.pdf

Add basic debugfs file to get API version.

Signed-off-by: Rajan Vaja <rajanv@xilinx.com>
Signed-off-by: Jolly Shah <jollys@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>