Changes to Freescale ATM drive (FUA) to improve mixed UBR/CBR rates

- Cells Per Slot (CPS) changed from 1 to 4, which reduces the slots to 296/4 = 74
- Default UBR rate reverted to 0.140
FUA driver fetches ATM MAC address from flattened device tree
Freescale ATM driver changes to prevent packet delays

- Increased the number of receive buffers
- Disabled interrupts on individual buffer descriptor completion
- Process all available buffer descriptors when interupted for a Frame complete
- Ensure buffers and skb are released on failure code paths