soc: nordic_nrf: fix usb delete statement location

/delete-node/ pointing at node labels needs to be out of the the tree
hierarchy, fixes the error:

devicetree error: zephyr/dts/arm/nordic/nrf52840_qfaa.dtsi:24 (column
16): parse error: expected node name

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>