mtd: Add an helper to make erase request aligned on ->erasesize

There's currently nothing forcing alignment of einfo->addr and
einfo->len on mtd->erasesize. Since we don't know if automatically
aligning those field in mtd_erase() will hurt some drivers, we add an
helper function to let drivers that need such an alignment explicitly
ask for it.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Miquel Raynal <miquel.raynal@free-electrons.com>