mpc85xx: use the generic board detect

mpc85xx: use the generic board detect

Use the values populated by the generic board detect function. The
first compatible from the device tree source file will be the board
name in userspace. The model property from the device tree source file
will be the model name.

Change the board name where used in the userspace and drop the target
specific board detect, to use the generic one.

Signed-off-by: Mathias Kresin <dev@kresin.me>