ppc4xx: Fix i2c divisor calcularion for PPC4xx

This patch fixes changes the i2c_init(...) function to use the function
get_OPB_freq() rather than calculating the OPB speed by
sysInfo.freqPLB/sysInfo.pllOpbDiv. The get_OPB_freq() function is
specific per processor. The prior method was not and so was calculating
the wrong speed for some PPC4xx processors.

Signed-off-by: Jeffrey Mann <mannj@embeddedplanet.com>
Signed-off-by: Stefan Roese <sr@denx.de>