Documentation: xilinx: Add documentation for eemi APIs

Add documentation for embedded energy management interface (EEMI)
APIs. It includes information about eemi ops and how to use them.
It also includes API information and supported IOCTL IDs which can
be used for device and control configuration.

Signed-off-by: Rajan Vaja <rajan.vaja@xilinx.com>
Signed-off-by: Jolly Shah <jollys@xilinx.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>