MIPS: ralink: mt7621: Fix soc_device introduction

Depending on selected SMP config options soc_device didn't get
initialised at all. With UP config vmlinux didn't link because
of missing soc bus.

Fixes: 71b9b5e0 ("MIPS: ralink: mt7621: introduce 'soc_device' initialization")
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Tested-by: René van Dorst <opensource@vdorst.com>