xilinx: zynqmp: remove chip_id function

Remove chip_id function and integrate the firmware call in the
zynqmp_get_silicon_idcode_name function. The change avoids querying the
firmware twice and makes the code bit more clear.

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