clk: tegra: Fix xusb_fs_src mux

The parent-to-index mapping for xusb_fs_src is incorrect.
Fix it by adding a mux table.

Signed-off-by: Jim Lin <jilin@nvidia.com>
Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>