rockchip: clk: rk3399: handle set_rate/get_rate for PLL_PPLL

The device-tree node for the PMU clk controller assigns to its parent
(i.e. PLL_PPLL) even though this clock currently is set up statically
by an init-function.

In order to avoid unexpected failures, a simple implementation of
set_rate (which accepts requests, but notifies the caller of the
preset frequency in its return value) and get_rate (which always
returns the preset frequency) are added.

Note that this is required for the RK808 PMIC to probe successfully on
the RK3399-Q7, following the support for the assigned-clocks property.

References: commit f4fcba5c ("clk: implement clk_set_defaults()")
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>