arm64: dts: juno: remove motherboard USB node

The first batch of Juno boards included a discrete USB controller chip
as a contingency in case of issues with the USB 2.0 IP integrated into
the SoC. As it turned out, the latter was fine, and to the best of my
knowledge the motherboard USB was never even brought up and validated.

Since this also isn't present on later boards, and uses a compatible
string undocumented and unmatched by any driver in the kernel, let's
just tidy it away for ever to avoid any confusion.

Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>