AR6MXS/Q: Add board_rev() function to detect board_id and assign IOMUX to UART2.
1. The weak internal pull-up makes board_id of rev 0.3 equals to 0xF.
2. Fix error "initialise statics to 0 or NULL" on emmc_en
AR6MXS/Q: fix WM8960 sysclk rate.
AR6MXS/Q: Add USB OTG ID and POWER function