clk: rockchip: only enter pll slow-mode directly before reboots on rk3288

As commit 1d33929e ("clk: rockchip: switch PLLs to slow mode before
reboot for rk3288") states, switching the PLLs to slow-mode is only
necessary when rebooting using the soft-reset done through the CRU.

The dwc2 controllers used create really big number of interrupts in
special constellations involving usb-hubs and their number is so high,
it can even overwhelm the interrupt handler if the cpu-speed os to low.

Right now the PLLs are put into slow-mode in a shutdown syscore_ops
callback which means it happens on all reboots (not only the soft-reset
ones) and even on poweroff actions.

This can result in the system not powering off and getting stuck instead,
so we should move the slow-mode change nearer to the actual reboot action.

For this we introduce the possiblity to also set a callback that gets
called from the restart-handler directly prior to restarting the system
and move the shutdown-callback to this new option.

With this the slow-mode switch is done only on the necessary reboots
and also has a smaller possibility of causing artifacts.

Fixes: 1d33929e ("clk: rockchip: switch PLLs to slow mode before reboot for rk3288")
Signed-off-by: Heiko Stuebner <heiko.stuebner@collabora.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>