arm64: dts: add MUSB node to Allwinner A64 dtsi

Allwinner A64 SoC has a MUSB controller like the one in A33, so add
a node for it, just use the compatible of A33 MUSB.

Host mode is tested to work properly on Pine64 and will be added into
the device tree of Pine64 in next patch.

Peripheral mode is also tested on Pine64, by changing dr_mode property
of usb_otg node and use a non-standard USB Type-A to Type-A cable.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>