arm: mvebu: a38x: Allocate bb_miiphy using bb_miiphy_alloc() and fill in callbacks

arm: mvebu: a38x: Allocate bb_miiphy using bb_miiphy_alloc() and fill in callbacks

Allocate bb_miiphy using bb_miiphy_alloc() and fill in callbacks
currently listed in bb_miiphy_buses[] array. This is a temporary
duplication of assignment to avoid breakage, which will be removed
in follow up patches. At this point, the bb_miiphy callbacks can
reach these accessors by doing container_of() on struct mii_dev.

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