powerpc/mpc85xx: Fix the I2C bus speed error on p1022

The source clock frequency of I2C bus on p1022 is the platform(CCB)
clock, not CCB/2. The wrong source clock frequency leads to wrong
I2C bus speed setting. so, fixed it.

Signed-off-by: Tang Yuantian <Yuantian.Tang@freescale.com>