clk: mediatek: Add USB clock support in MT8173 APMIXEDSYS

Add REF2USB_TX clock support into MT8173 APMIXEDSYS. This clock
is needed by USB 3.0.

Signed-off-by: James Liao <jamesjj.liao@mediatek.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>