Move malloc_cache_aligned() to its own header

At present malloc.h is included everywhere since it recently was added to
common.h in this commit:

   4519668b mtd/nand/ubi: assortment of alignment fixes

This seems wasteful and unnecessary. We have been trying to trim down
common.h and put separate functions into separate header files and that
change goes in the opposite direction.

Move malloc_cache_aligned() to a new header so that this can be avoided.
The header would perhaps be better named as alignmem.h but it needs to be
included after common.h and people might be confused by this. With the name
memalign.h it fits nicely after malloc() in most cases.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>