ARM: pl01x debug code endian fix

The PL01X debug code needs to take into account which endian mode the
processor is running in. If it is big-endian, ensure the data is swapped
appropriately.

Note, we could do this slightly more efficiently if we have an macro to
do the necessary swap for the bits used by test.

Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>