xilinx: zynqmp: get chip ID at EL3

Modify the board init function to allow getting the chip ID when U-Boot
proper is executed at EL3.

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