board: phytec: introduce eeprom struct member 'valid'

Add a new nember to the eeprom_data that indicates whether the
associated data is valid or not. Make use of this new member in the
phytec_eeprom_data_init function by setting the valid value
appropriately.
Move the eeprom data to a new struct payload that holds
the payload of the eeprom.

Signed-off-by: Yannic Moog <y.moog@phytec.de>
Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
Reviewed-by: Teresa Remmet <t.remmet@phytec.de>