USB: Add dcache support to the EHCI driver

This patch adds routines to handle (flush/invalidate) the dcache for the
QH and qTD structures and data buffers. This is needed on platforms using
this EHCI support with dcache enabled (like the MIPS VCT board port).

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Remy Bohmer <linux@bohmer.net>