powerpc/mpc8xxx: Set inactive csn_bnds to 0xffffffff

When chip select interleaving is enabled, cs0_bnds is used for address
binding. Other csn_bnds are not used. When two controllers interleaving is
enabled, cs0_bnds of both controllers are used, other csn_bnds are not.
However, the unused csn_bnds may be used internally for calculating
addresses for calibration. Setting those registers to 0 may confuse
controllers in some cases. Instead, setting them to 0xffffffff together
with normal LAWs will guarantee the address is not mapped to DDR.

Signed-off-by: York Sun <yorksun@freescale.com>