Signed-off-by: Peter Tyser <ptyser@xes-inc.com>

The CPUDIR variable points to the location of a target's CPU directory.
Currently, it is set to cpu/$CPU.  However, using $CPUDIR will allow for
more flexibility in the future.  It lays the groundwork for reorganizing
U-Boot's directory structure to support a layout such as:

  arch/$ARCH/cpu/$CPU/* (architecture with multiple CPU types)
  arch/$ARCH/cpu/*      (architecture with one CPU type)

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>

- Introducing the concept of SoCs "./cpu/$(CPU)/$(SOC)"
- creating subdirs for SoCs ./cpu/arm920t/imx and ./cpu/arm920t/s3c24x0
- moving SoC specific code out of cpu/arm920t/ into cpu/arm920t/$(SOC)/
- moving drivers/s3c24x0_i2c.c and drivers/serial_imx.c out of drivers/
  into cpu/arm920t/$(SOC)/