mpc5200: digsy_mtc: fix extension board EEPROM I2C address for rev5

On newer rev5 hardware the extension board EEPROM I2C address
has been changed to 0x54. Make this I2C address configurable
depending on CONFIG_DIGSY_REV5 so that extention board presence
detection works correctly on newer hardware.

Signed-off-by: Anatolij Gustschin <agust@denx.de>