arm64: Fix the endianness of arch_spinlock_t

The owner and next members of the arch_spinlock_t structure need to be
swapped when compiling for big endian.

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