V4L/DVB (12416): radio-si470x: add i2c driver for si470x

This patch supports i2c interface of si470x. The i2c specific part
exists in radio-si470x-i2c.c file and the common part uses
radio-si470x-common.c file. The '#if defined' is inserted inevitably
because of parts used only si470x usb in the common file.

The current driver version doesn't support the RDS.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>