arm64: dts: rockchip: Add pinctrl entry for 32k clock on rk3399

On some rk3399 boards GPIO0_A0 is hooked up to a 32 kHz clock.  This can
be used as the source for various clocks in the system.

Add a pinmux so boards can get this pin properly configured.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>