dwmmc: Add clock support and other improvements

• If compiled with EXT_RESOURCES look up the "biu" and "ciu" clocks in the DT
• Don't use custom property "bus-frequency" but the standard one "clock-frequency"
• Use the DT property max-frequency and fall back to 200Mhz if it don't exists
• Add more mmc caps suported by the controller
• Always ack all interrupts
• Subclassed driver can supply an update_ios so they can handle update the clocks accordingly
• Take care of the DDR bit in update_ios (no functional change since we do not support voltage change for now)
• Make use of the FDT bus-width property