imx6: ensure AHB clock is 132MHz in low freq boot mode

imx6: ensure AHB clock is 132MHz in low freq boot mode

For low freq boot mode(ARM boot up with 396MHz), ROM
will not set AHB clock to 132MHz, and the reset value of
AHB divider is incorrect which will lead to wrong AHB
rate, need to correct it. To enable low freq boot mode,
need to set BOOT_CFG2[2] to high, tested on i.MX6Q/DL
SabreSD board and i.MX6SL EVK board.

Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Anson Huang <b20788@freescale.com>