A64 SoC have a CCU (r_ccu) in PRCM block.

Add the device node for it.

The mux 3 of R_CCU is an internal oscillator, which is 16MHz according
to the user manual, and has only 30% accuracy based on our experience
on older SoCs. The real mesaured value of it on two Pine64 boards is
around 11MHz, which is around 70% of 16MHz.
Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>