Initial dts support AR6MXQ

1. Disable RTC s35390a.
Initial ar6mx_defconfig
Enable power to AU6256 ports USB-1 and USB-2 and remove keep-power-in-suspend for USDHC3.
Enable LED support in the kernel and configure defaults.

1. status LED (orange) set to heartbeat by default.
2. user LED (green) is turned off.