plat: zynqmp: Don't panic() if we can't find the FSBL struct

plat: zynqmp: Don't panic() if we can't find the FSBL struct

If we can't find the FSBL handoff struct don't panic and just use the
defaults instead.

We still print a warning to the user to let them know what we couldn't
find it.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>