powerpc/8xxx: Remove dependency on <usb.h>

We used <usb.h> for USB_MAX_DEVICE.  However this requires we actual
build in support for USB into u-boot (which should not be required for
device tree fixup).

At this time no FSL SoC that utilizies this code (83xx/85xx) has more
than 2 USB controllers.  So we replace USB_MAX_DEVICE with a local
define FSL_MAX_NUM_USB_CTRLS.

If/when a device shows up with more than 2 controllers we can easily
bump this value or refactor into a proper define per SoC.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>