ramips: mt7621: introduce 'soc_device' initialization

ramips: mt7621: introduce 'soc_device' initialization

mt7621 SoC has its own 'ralink_soc_info' structure with some
information about the soc itself. Pcie controller and pcie phy
drivers for this soc which are still in staging git tree make uses
of 'soc_device_attribute' looking for revision 'E2' in order to
know if reset lines are or not inverted. This way of doing things
seems to be necessary in order to make things clean and properly.
Hence, introduce this 'soc_device' to be able to properly use those
attributes in drivers. Also set 'data' pointer points to the struct
'ralink_soc_info' to be able to export also current soc information
using this mechanism.

Signed-off-by: DENG Qingfang <dengqf6@mail2.sysu.edu.cn>