mpc83xx: Fix the SATA clock setting of 837x targets

Currently the SATA controller clock is configured as CSB clock,
usually the CSB clock is 400/333/266MHz.

However, The SATA IP block is only guaranteed to operate up to
200 MHz as stated in the HW spec.

The bug is reported by Joe D'Abbraccio <ljd015@freescale.com>

This patch makes the SATA clock as half of CSB clock.

Signed-off-by: Dave Liu <daveliu@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>