powerpc: mpc85xx: Allow boards to override CONFIG_USB_MAX_CONTROLLER_COUNT

If the board isn't strapped to enable USB1 then attempting to access it
will result in a hang. Avoid this by allowing boards to define
CONFIG_USB_MAX_CONTROLLER_COUNT.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>