dt-bindings: usb: amlogic, meson-g12a-usb-ctrl: fix clock names

dwc2 bindings require clock-names to be "otg".
Fix the example in amlogic,meson-g12a-usb-ctrl to follow this requirement.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Rob Herring <robh@kernel.org>