mpc5200: digsy_mtc: Fix extension board detection

mpc5200: digsy_mtc: Fix extension board detection

Switch to extension board detection using pci_find_device()
instead of detecting by i2c access to EEPROM device on
extension board.

This is a cleaner detection method since EEPROM addresses
can be different on different board revisions. This also
avoids "i2c_read: failed to address chip" error messages
in the boot log on boards without extension board which
may confuse users.

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