General code modification for mpc7448hpc2 board support.

1. Add 7447A and 7448 processor support.
2. Add the following flags.

CFG_CONFIG_BUS_CLK : If the 74xx bus frequency can be configured dynamically
(such as by switch on board), this flag should be set.

CFG_EXCEPTION_AFTER_RELOCATE: If an exception occurs after the u-boot
relocates to RAM, this flag should be set.

CFG_SERIAL_HANG_IN_EXCEPTION: If the print out function will cause the
system hang in exception, this flag should be set.

There is a design issue for tsi108/109 pci configure  read. When pci scan
the slots, if there is no pci card, the tsi108/9 will cause a machine
check exception for mpc7448 processor.

Signed-off-by: Alexandre Bounine <alexandreb@tundra.com>
Signed-off-by: Roy Zang <tie-fei.zang@freescale.com>