Fixed driver for SystemACE

The current SystemACE driver did not work while testing U-Boot on the ML605 board.
These changes should fix some issues regarding the old implementations and works for me.
The following changes are made:
 * Added definitions of registers and its contents
 * Changed functions for read/write and or/and of registers
   - Fixed wrong endianess in read/write functions (SystemACE always operates in little endian mode)
   - Added 16 and 32 bit functions since only 16bit word access is not sufficient

This driver is tested with the Microblaze v8.50.c attached to the AXI Bus on an ML605 board.