powerpc/atomic: Loosen the memory barrier on atomic_load_acq_*()

'sync' is pretty heavy-handed, and is unnecessary for this use case. It's a
full barrier, which is applicable for all storage types. However,
atomic_load_acq_*() is only expected to operate on physical memory, not
device memory, so lwsync is sufficient (lwsync provides access ordering on
memory that is marked as Coherency Required and is not Write Through nor
Cache Inhibited). On 32-bit systems, this is a nop, since powerpc_lwsync()
is defined to use sync, as a workaround for a silicon bug in the Freescale
e500 core.