xilinx: zynqmp: merge firmware calls for EL2 and EL3

This patch merges ZynqMP firmware calls under xilinx_pm_request in order
to make trainsparent the EL. Calls at EL3 are send through IPI messages
and EL2 through SMC calls.

The EL2 call uses fixed payload and arg size as the EL3 call. The
firmware is capable to handle PMUFW_PAYLOAD_ARG_CNT bytes but the
firmware API is limited by the SMC call size.

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