ramips: adding registration for si3210

The si3210 is a SLIC device providing a complete analog telephone
interface and therefore frequently used in soho router.
The si3210 have a native spi interface to be controlled by the CPU
but currently there is no dedicated driver in lede.
Adding a registration for this device in spidev allow to control the
device in user space.
This way of patching is also in line with the rationale of the spidev
driver, see: http://marc.info/?t=148145791900001&r=1&w=2


The si3210 has been also added in the DWR-512 DT to properly describe
the HW.

Signed-off-by: Giuseppe Lippolis <giu.lippolis@gmail.com>