CHROMIUM: usb: dwc3: allow to configure the XHCI to use the internal clock

CHROMIUM: usb: dwc3: allow to configure the XHCI to use the internal clock

Remove the hardcoded assumption that we have an external PLL for the USB3 clock.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
BUG=chrome-os-partner:16351
TEST=on Spring, plug on usb key on USB3 port and browse the content.

Change-Id: I0bf2a3604b3009abbcdfe3b0daf9d7fcccf1438c
Reviewed-on: https://gerrit.chromium.org/gerrit/38684
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Todd Broch <tbroch@chromium.org>
Reviewed-by: Olof Johansson <olofj@chromium.org>
Commit-Ready: Vincent Palatin <vpalatin@chromium.org>