ENGR00180919 [MX6]Update clock tree if BUS freq is changed

ENGR00180919 [MX6]Update clock tree if BUS freq is changed

As DDR freq change is by modifying CCM register directly,
we need to update the clock tree as well, or the clock
tree will be broken. Also, we need to make sure the clock
rate counting is right.

Signed-off-by: Anson Huang <b20788@freescale.com>