Fix the exception occuring in RAM table search issue.

The original search_one_table() function code can only processes the search
for the exception occurring in FLASH/ROM, because the exception and fixup
table usually locate in FLASH. If the exception address is also in
FLASH, it will be OK.
If the exception occurs in RAM, after the u-boot relocation, a
relocation offset should be added.

clean up the code in cpu/74xx_7xx/cpu.c

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