net: ethernet: ti: cpsw_ale: Add cpsw_ale_restore() helper

This can be used by device driver to restore ALE context.
The data produced by cpsw_ale_dump() can be passed to
cpsw_ale_restore().

This is required as on certain platforms the ALE context
is lost on low power suspend/resume.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>