powerpc/8xxx: Update USB mode device tree fixup

Modify support for USB mode fixup:
        - Add common support for USB mode and phy type
          device tree fix-up for all USB controllers
          mentioned in hwconfig string
        - Fetch USB mode and phy type via hwconfig; if not
          defined in hwconfig, then fetch them from env

Signed-off-by: Ramneek Mehresh <ramneek.mehresh@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>