powerpc: use asm-generic/unaligned.h

Powerpc configurations are apparently able to do unaligned accesses. But
in an attempt to clean up and handle unaligned accesses in the same way
we ignore that and use the common asm-generic/unaligned.h directly
instead.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>