CHROMIUM: usb: allow to de-activate some ports on EHCI root hub

CHROMIUM: usb: allow to de-activate some ports on EHCI root hub

To optimize power consumption and sometimes avoid electrical conflicts
on the board, we might want to avoid activating some of the ports on
the root hub.
The platform specific part of the driver can optionally provide a bitmap
of the enabled ports. By default, all ports are enabled.

This might be reverted once we have another mechanism to control VBUS
regulator on the next hardware revision.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
BUG=chrome-os-partner:14490
TEST=boot on Spring, verify that port 0 regulator is not enabled and
port 1 is detecting a USB key correctly.
boot on Snow, verify that port 0 is detecting a USB key

Change-Id: Ib08d18a105e366b37f5117a6f7c0e1e56e04dce5
Reviewed-on: https://gerrit.chromium.org/gerrit/38222
Commit-Ready: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Olof Johansson <olofj@chromium.org>