usb: xhci: plat: Enable async suspend/resume

USB host controllers can take a significant amount of time to suspend
and resume, adding several hundred miliseconds to the kernel resume
time. Since the XHCI controller has no outside dependencies (other than
clocks, which are suspended late/resumed early), allow it to suspend and
resume asynchronously.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Tested-by: Andrew Bresticker <abrestic@chromium.org>
Tested-by: Robert Foss <robert.foss@collabora.com>
Signed-off-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>