lib/genalloc.c: add power aligned algorithm

One of the more common algorithms used for allocation is to align the
start address of the allocation to the order of size requested.  Add this
as an algorithm option for genalloc.

Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Olof Johansson <olof@lixom.net>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: David Riley <davidriley@chromium.org>
Cc: Ritesh Harjain <ritesh.harjani@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>