New AR933x SoC registers definitions
Move ar7240_sys_frequency function to external, new file: ar933x_clocks.c
New and better AR933x UART driver, with support for baudrate switch, used it upstrem U-Boot version, not yet released for this SoC
Switch to new UART driver and allow to use more baudrates on Dragino2 board for test purposes. This changes break code on boards with compressed, 64 KB U-Boot image.