[POWERPC] bootwrapper: Fix array handling in dt_xlate_reg().

This fixes a few bugs in how dt_xlate_reg() handles address arrays:

1. copy_val() was copying into the wrong end of the array, resulting
in random stack garbage at the other end.
2. dt_xlate_reg() was getting the result from the wrong end of the array.
3. add_reg() and sub_reg() were treating the arrays as
little-endian rather than big-endian.
4. add_reg() only returned an error on a carry out of the entire
array, rather than out of the naddr portion.
5. The requested reg resource was checked to see if it exceeded
the size of the reg property, but not to see if it exceeded the
size of the buffer.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>