sb600: Implement EHCI workaround

sb600: Implement EHCI workaround

Linux implements it itself, but older Linuxes and other systems
might not. Without this, the host controller might not respond
to drivers.

Change-Id: I4ff0e3683c02e7aa00d188428847c64c4c5d589d
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/345
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>