clk: tegra: Fix xusb_hs_src clock hierarchy

Currently the Tegra1x4 clock init code hard-codes the mux setting
for xusb_hs_src and treats it as a fixed-factor clock.  It is,
however, a mux which can be parented by either xusb_ss_src/2 or
pll_u_60M.  Add the fixed-factor clock xusb_ss_div2 and put an
entry in periph_clks[] for the xusb_hs_src mux.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>