mips: ralink: mt7620: introduce 'soc_device' initialization

MT7620 SoCs have their own 'ralink_soc_info' structure with some
information about the soc itself. In order to be able to retrieve this
information from driver code and avoid architecture dependencies for
retrieving these details introduce this 'soc_device'. Set 'data' pointer
points to the struct 'ralink_soc_info' to be able to export also current
soc information using this mechanism. We need to select 'SOC_BUS' in
Kconfig configuration for these SoCs.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>