firmware: zynqmp: Enable IPI code calling also in EL3

U-Boot proper can still run in EL3 without using firmware interface wired
via ATF. For supporting this use case there is a need to check EL level
where U-Boot runs and based on that choose the way how to talk to firmware.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>