arm64: zynqmp: Modify chip_id routine to get either idcode or version

This patch modifies the chip_id routine to get either idcode or
silicon version based on the argument received.

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