arm64: dts: rockchip: Add 32k clk on rk3399-gru

This adds the 32k clock to the RK3399 Gru board file, which is provided
by a Silego oscillator on Gru boards.

Even though it's not directly used, muxes will end up traversing the
entire clk tree on calls to determine_rate if it doesn't exist. This
is because the 32k clk is listed as a possible parent on some clks.
Since the clk doesn't know about the 32k clk (it was never registered),
it triggers a global search for it. This can happen about 40 times per
second, which isn't great for power.

Signed-off-by: Derek Basehore <dbasehore@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
[moved clock position and adapted commit message a bit]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>