km/common: moved eeprom config to pbec specific part

Moved eeprom config to specific part, to allow bigger eeprom write pages
for km_kirkwood designs. Write page only used for env eeprom in std use
cases. 24C128 has page size of 64bytes -> 8 time faster.

Signed-off-by: Stefan Bigler <stefan.bigler@keymile.com>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <wd@denx.de>