USB: Implement support for EHCI with big endian MMIO

This patch implements supports for EHCI controllers whose MMIO
registers are big endian and enables that functionality for
the Toshiba SCC chip. It does _not_ add support for big endian
in-memory data structures as this is not needed for that chip
and I hope it will never be.

The guts of the patch are to convert readl(...) to
ehci_readl(ehci, ...) and similarly for register writes.

Signed-off-by: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Geoff Levand <geoffrey.levand@am.sony.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>