85xx: Using proper I2C source clock divider for MPC8544

Measurements with our MPC8544 board showed that the I2C bus frequency
is wrong by a factor of 1.5. Obviously, the interpretation of the
MPC85xx_PORDEVSR2_SEC_CFG bit of the cfg_sec_freq register is not
correct. There seems to be an error in the 8544 RM.

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>