arm64: zynqmp: probe firmware driver

Probe ZynqMP firmware driver on the board initialization phase and
ensure that firmware is in place to continue execution. The probing is
done on board_init so it can be used for both SPL and U-Boot proper.

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