xilinx: Move zynq_board_read_rom_ethaddr to shared location

Zynq and ZynqMP are sharing similar code and there is no reason to do
code duplication. Move zynq_board_read_rom_ethaddr() to common file for
easier conversion to DM.
Use ZynqMP version that's why also add CONFIG_ZYNQ_EEPROM_BUS to Syzygy
which is only one Zynq board which is using this feature.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>