mpc83xx: Add config of eTSEC emergency priority in SPCR

The TSEC emergency priority definition of 831x/837x
is different than the definition of 834x in SPCR register.

Add the other config of TSEC emergency priority into
cpu_init.c

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