ARM: dts: meson: add USB support on Meson8 and Meson8b

This adds the DWC2 USB controller nodes and the corresponding USB2 PHY
nodes to meson.dtsi (as the same - or at least a very similar) IP block
is used on all SoCs (at the same physical address).
Additionally meson8.dtsi and meson8b.dtsi add the required clocks to the
DWC2 and USB2 PHY nodes, otherwise the DWC2 controller cannot be
initialized by the dwc2 driver.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>