arm: imx: Enable lpddr3_freq_imx for imx6

For imx6 bsp, the SDK actually integrates imx6 config
and imx7 config, so lpddr3_freq_imx is needed when we
use imx6, otherwise it will cause a series of problems,
such as blurred graphics displayed by the display system,
etc., so in order to avoid turning on the CONFIG_SOC_IMX7D and
CONFIG_SOC_IMX7ULP options, and enable lpddr3_freq_imx on imx6.

Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>