spl/mpc85xx: move udelay to cpu code

It applies to non-Freescale 85xx boards as well as Freescale boards,
so it doesn't belong in board/freescale.  Plus, it needs to come out
of nand_spl if it's to be used by the new SPL.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: Andy Fleming <afleming@freescale.com>