xilinx: zynqmp: get chip ID using firmware driver

Current implementation for getting chip ID uses either raw access on EL3
or a SMC call to get the silicon information. Following change
simplifies the code using always the firmware driver.

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