drivers/usb/ehci: Use platform-specific accessors

Current implementation doesn't allow utilization of platform-specific
reads and writes.

But some arches or platforms may want to use their accessors that do
some extra work like adding barriers for data serialization etc.

Interesting enough OHCI accessors already do that so just aligning
EHCI to it now.

This is a resend of http://patchwork.ozlabs.org/patch/726714/
Back in the day this patch broke some PPC and Sandbox boards
as they we missing inclusion of "asm/io.h". Those missing items were
fixed with:
 1) http://patchwork.ozlabs.org/patch/751397/
 2) http://patchwork.ozlabs.org/patch/771099/

So now it should be safe to apply this patch.
FWIW TravisCI builds everything with all 3 patches in place,
see https://travis-ci.org/abrodkin/u-boot/builds/239563813



Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Acked-by: Marek Vasut <marex@denx.de>