85xx: Update MPC85xx_PORDEVSR_IO_SEL mask

The MPC8572 has a 4-bit wide PORDEVSR IO_SEL field. Other MPC85xx
processors have a 3-bit wide IO_SEL field but have the most
significant bit is wired to 0 so this change should not affect
them.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>