firmware: zynqmp: create firmware header

New firmware header to place firmware specific macro and function
declarations. The patch also moves the macros defining PM operations as
well as some helper macros.

Signed-off-by: Ibai Erkiaga <ibai.erkiaga-elorza@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>