clk: tegra: Initialize DSI low-power clocks

clk: tegra: Initialize DSI low-power clocks

The low-power DSI clocks are used during host-driven transactions on the
DSI bus. Documentation recommends that they be children of PLLP and run
at a frequency of at least 52 MHz.

Signed-off-by: Thierry Reding <treding@nvidia.com>