arm64: zynqmp: Make chip_id a global routine()

This patch makes chip_id() as a global routine so that
it can be used in other places as required.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>