powerpc/85xx: get rid of enum board_slots in P4080 MDIO driver

enum board_slots contained six values, where SLOT1 == 1, SLOT2 == 2, and
so on.  This is pointless, so remove it.  Also move the lane_to_slot[]
array to the top of the file so that it can be used by other functions.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>