USB: shutdown USB before booting

This patch fixes a potentially serious issue related to USB which was
discouvered by Martin Krause <martin.krause@tqs.de> and fixed for
ARM920T. Martin wrote:

  Turn off USB to prevent the host controller from writing to the
  SDRAM while Linux is booting. This could happen, because the HCCA
  (Host Controller Communication Area) lies within the SDRAM and the
  host controller writes continously to this area (as busmaster!), for
  example to increase the HccaFrameNumber variable, which happens
  every 1 ms.

This is a slightly modified version of the patch in order to shutdown
USB when booting on all architectures.

Signed-off-by: Markus Klotzbuecher <mk@denx.de>