ramips: add missing reset button for Nexx WT1520

ramips: add missing reset button for Nexx WT1520

This commit adds missing the GPIO key used as reset button.
Nexx WT1520 has a GPIO key for factory reset, but it's not defined in
WT1520.dtsi and cannot use it.

Drop the UART (full) from the device tree source file, it was never
used for this board. Adjust the kernel bootargs accordingly.

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
[add note about dropped UART (full) to the commit message]
Signed-off-by: Mathias Kresin <dev@kresin.me>