arm64: debug: make aarch32 bkpt checking endian clean

The current breakpoint instruction checking code for A32 is not endian
clean. Fix this with appropriate byte-swapping when retrieving
instructions.

Signed-off-by: Matthew Leach <matthew.leach@arm.com>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>