xilinx: board: Read the whole eeprom not just offset

Starts to use new way how eeproms should be referenced.
Reference is done via nvmem alias nodes. When this new way is specified
code itself read the eeprom and decode xilinx legacy format and fill struct
xilinx_board_description. Then based on information present there board_*
variables are setup.
If variables are saved and content can't be changed information is just
shown on console.

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