dt: bindings: add binding for Allwinner A64 R_PIO pinctrl

Allwinner A64 SoC has also a dedicated pin controller for Port L GPIOs,
which is called "Port Controller (CPUs-PORT)" in SoC User Manual.

Add a binding for this pin controller, like the ones in A23/33 and H3.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>