net: miiphybb: Introduce bb_miiphy_alloc()/bb_miiphy_free() wrappers

net: miiphybb: Introduce bb_miiphy_alloc()/bb_miiphy_free() wrappers

Introduce bb_miiphy_alloc()/bb_miiphy_free() wrappers to allocate and free
struct bb_miiphy_bus. Make struct bb_miiphy_bus wrap struct mii_dev, which
will become useful later in bb_miiphy_bus accessors, which would be able
to access struct bb_miiphy_bus using container_of, even if the PHY stack
only passes in the inner struct mii_dev .

Reviewed-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>