ramips: cleanup AsiaRF AWM002 eval board support

ramips: cleanup AsiaRF AWM002 eval board support

Cleanup the dtsi files and remove one layer of dtsi. Set the size of
the firmware partition to a value matching the flash size from the
board (variant) name.

Remove the usb led trigger. There is neither a default config for the
usb led trigger nor a LED for usb activity indication.

Signed-off-by: Mathias Kresin <dev@kresin.me>