kexec: ppc: use _ALIGN* to make the logic clear

By replacing all the explicit align opertion with marco _ALIGN*,
the code logic could be more clear.

Besides, remove the duplicate _ALIGN_* definition in file
kexec/arch/ppc/crashdump-powerpc.h.

Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Signed-off-by: Simon Horman <horms@verge.net.au>