ARM: traps: use <asm/opcodes.h> to get correct instruction order

The trap handler needs to take into account the endian configuration of
the system when loading instructions. Use <asm/opcodes.h> to provide the
necessary conversion functions.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>