xilinx: common: Move ZYNQ_GEM_I2C_MAC_OFFSET to board Kconfig

xilinx: common: Move ZYNQ_GEM_I2C_MAC_OFFSET to board Kconfig

There is no reason to have ZYNQ specific Kconfig macro in generic location
to be visible for all other SoCs. That's why move it to Xilinx common
location to be visible only for us.
Also introduce new bool entry ZYNQ_MAC_IN_EEPROM to have also an option to
disable it or enable. This has connection to code which is reading the
whole content of i2c and also work with the rest of date not just with MAC
address.

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