mpc83xx: Add support for the display of reset status

83xx processor family has many reset sources, such as
power on reset, software hard reset, software soft reset,
JTAG, bus monitor, software watchdog, check stop reset,
external hard reset, external software reset.
sometimes, to figure out the fault of system, we need to
know the cause of reset early before the prompt of
u-boot present.

Signed-off-by: Dave Liu <daveliu@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>