xilinx: zynqmp: refactor silicon name function

Current algorithm used to get the silicon name is bit complicated and
hard to follow. Updated to use more straightforward mechanism based on
the Device ID code table (Table 1-2). The full IDCODE register is used
(except device revision bits [31:28]) to get the device name and IDCODE2
value is used for identifying the variant.

Additionally to make the algorithm bit more clear it also save some space
as the devices table is slightly bit smaller.

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