clk: rockchip: add a type from SGRF-controlled gate clocks

Some clk gates on Rockchip SoCs are part of the SGRF (secure general
register files) and thus only controllable from secure mode, with the
most prominent example being the watchdog.

In most cases we still want to define this as a real clock though,
to have complete clock tree and not reference the generic base-clock
from the devicetree.

So far we've just defined this as factor-1-1 clocks in the clock init,
so define a special clock-type for it so that this definition can be
part of the general tree-definition and save some boilerplate code.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>