clk: rockchip: register pll mux before pll itself

The structure is xin24m -> pll -> pll-mux (xin24m,pll,xin32k). The pll
does have an init callback to make sure the boot-selected frequency is
using the expected pll settings and resets the same frequency using
the values provided in the driver if necessary.

The setting itself also involves remuxing the pll-mux temporarily to
the xin24m source to let the new pll rate settle. Until now this worked
flawlessly, even when it had the flaw of accessing the mux settings
before the mux actually got registered.

With the recent clock-core conversions this flaw became apparent in
null pointer dereference in
[<c03fc400>] (clk_hw_get_num_parents) from [<c0400df0>] (clk_mux_get_parent+0x14/0xc8)
[<c0400ddc>] (clk_mux_get_parent) from [<c040246c>] (rockchip_rk3066_pll_set_rate+0xd8/0x320)

So to fix that, simply register the pll-mux before the pll, so that
it will be fully initialized when the pll clock executes its init-
callback and possibly touches the pll-mux clock.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>