ppc4xx: Reconfigure PLL for 667MHz processor for PPC440EPx

On PPC440EPx without a bootstrap I2C EEPROM, the PLL can be reconfigured
after startup to change the speed of the clocks. This patch adds the
option CFG_PLL_RECONFIG. If this option is set to 667, the CPU
initialization code will reconfigure the PLL to run the system with a CPU
frequency of 667MHz and PLB frequency of 166MHz, without the need for an
external EEPROM.

Signed-off-by: Mike Nuss <mike@terascala.com>
Acked-by: Stefan Roese <sr@denx.de>