MIPS: ralink: mt7621: avoid to init common ralink reset controller

Commit 38a8553b ("clk: ralink: make system controller node a reset provider")
make system controller a reset provider for mt7621 ralink SoCs. Ralink init code
also tries to start previous common reset controller which at the end tries to
find device tree node 'ralink,rt2880-reset'. mt7621 device tree file is not
using at all this node anymore. Hence avoid to init this common reset controller
for mt7621 ralink SoCs to avoid 'Failed to find reset controller node' boot
error trace error.

Fixes: 64b2d6ff ("staging: mt7621-dts: align resets with binding documentation")
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>