staging: mt7621-dts: add gpio perst to pcie bindings node

Driver has been changed to use perts gpio reset instead of
perst builtin behaviour. Update device tree accordly.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>