arm64: rockchip, implement the two rk805/808 clocks

While the xin32k clk was implemented in rk3399_cru as a fixed rate
clock, migrate it to rk805 as we will also need the 2nd clock
'rtc_clko_wifi' for SDIO and BT.
Both clocks remain fixed rate, and while the 1st one is always on
(though that is not expressed in the clk framework), the 2nd one
we can toggle on/off.

Reviewed-by: manu
Tested-by: manu
MFC-after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D26870